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Lesson 7 – Plastic Repair
Like It Or Not, It’s Here To Stay
The world hates change, yet it is the only thing that has brought progress. ~Charles Kettering
In this lesson we are going to cover why plastics started being used on cars, the two types of plastic, and repair methods to repair plastic.
This page is the full lesson, which includes text content, videos and resources. I also broke it down to only the videos and resources. Just click the button below if you prefer to view only the videos or resources. Note: some browsers and mobile devices may not play videos below…but they should work on the videos page. If you are still having problems viewing videos, then you can watch the playlist on YouTube.
I don’t currently have a video overview of this lesson. Check back soon. I do have demonstration videos below demonstrating how to repair plastic.
When plastic first started being produced on cars, we would just replace most plastic parts when damaged, but that time has passed. As more plastics are being used on cars advanced repair methods and products have been developed as well. With all of the plastic on cars these days, we need to make sure technicians know how to use the new products to properly make the repairs.
Plastic Repair – It’s A Different World
The collision repair industry is changing fast. Not too long ago cars consisted of a lot of metal. Mostly interior parts were made from plastic, but even much of the inside of a car was made with metal. In the past 15 to 20 years, plastics and composites have became widely used for interior, exterior, body parts, mechanical parts, etc. Plastic is not going any where, so we need to learn how to repair it properly.
The government started pushing car makers to make cars more fuel efficient. One obvious solution was to make the car lighter. This is when car makers started working with metal and other materials to reduce weight in vehicles. Lighter metal started being designed, such as high strength steel to allow the metal to have equivalent strength with much thinner and lighter metal. At the same time, engineers started brainstorming and started using light weight plastic for many of the parts that were made of steel in the past. Apparently, the results were positive, as there is more plastic in each new body style of cars.
More Fuel Economy
To this day the government is requiring car makers to meet certain requirements in order to sell cars in the US. Therefore, the automakers are going to do what ever it takes to make this happen. Therefore, I believe we will see more plastic, aluminum, carbon fiber, and other composites in the years to come.
Now that the Department of Transportation and the EPA have locked in the new fuel efficiency of 54.5 mile per gallon by 2025, I believe there will be a push to increase the use of plastics, composites, and aluminum to lighten cars. We are currently at 27.5 MPG so the auto manufactures have a lot of work to do over the next 13 years. I know, you may be thinking more economical cars will be produces, but the vehicles will need to sell too! Customers demand all of the bells and whistles, room and luxury. To meet the demands of the customers and the required MPG rules, who knows what may be coming our way.
Here is an article with more information about the new 54.5 MPG rules. http://www.cnbc.com/id/48829545/Getting_to_54_MPG_Will_Change_What_We_Drive
Like It Or Hate It
There are many technicians who do not like working with plastics. However, whether we like it or not, the fact that it is here to stay is a pretty safe bet. I am not trying to promote the use of plastic, but we do need to learn how to repair it to stay competitive with the industry.
One of the first methods to repair plastic was plastic welding. This involved identifying the type of plastic, then selecting the correct method to weld the plastic. This method is still used, but adhesives are preferred by most body shops.
One way to determine the type of plastic on a part is to look for International Organization for Standardization (ISO) stamped on the back side of the part. However, these codes are not located in the same place, which makes it hard to find the plastic type. Some parts may not even have the the ISO on it.
If you are not able to find the ISO, it is difficult to determine what kind of plastic it is. Plastic may also consist of a blend of plastics. If the plastic is a blend of plastics will make the process of selecting the best plastic rod difficult to determine.
Here is a link to help identify the plastic type
There are some tests that can be performed that will help determine the type of plastic, which we will discuss below.
Two Plastic Types
All plastic falls into two types theromset and thermoplastic.
Thermoset plastic: (wikipedia) is basically a product that consists of several parts together that become one. In other words, thermoset plastic changes chemical properties during the curing process. Like body filler, once the two products are mixed together and dried, it is now a different product. You are not going to be able to separate the body filler and hardener again. One of the things that you need to remember about thermoset plastic is that it does not reshape easily, if at all, with heat. This plastic type can not be repaired by the welding method. The only way this type of plastic can be repaired is by using adhesive, which we will discuss shortly.
Thermoplastics: (wikipedia) are more flexible and do not take a chemical change during production of the part. It is kind of like latex paint. It is the same product dry or wet. In the dry state it simply means that the water has evaporated from the paint. Therefore, the two products are separated after they were mixed together.
Thermoplastic is easy to reshape with applied heat. Thermoplastic can be repaired using the plastic repair method and by using adhesives. However, many thermoplastics may have polyofins, which create a challenge with adhesion. Therefore, adhesion promoters must be used when repairing or painting plastics with polyofins in them. To be safe, I normally use adhesion promoter on all plastics to ensure adequate adhesion.
Bumper covers are usually a thermoplastic and the most common plastic part needing a repair on cars.
Flexible Plastic and Rigid Plastic
Plastic is also classified according to how flexible or rigid the plastic is. As mentioned, most plastic repaired on cars are bumper covers, which are more on the flexible side. However, there have been cars produced with every exterior panel made with plastic, such as the Saturn, which consists of flexible and rigid plastic.
Simplified repair methods have been developed using adhesives. By using adhesives to repair plastic, you must determine if the plastic is flexible, semi-flexible, semi-rigid, or rigid. Then simply select the repair adhesive to repair the plastic. This is much easier than identifying the plastic type and selecting the correct rod to weld plastic. Therefore, the focus of this guide is going to be using adhesives to repair plastic parts.
Adhesive Plastic Repair
With all the plastics, companies such as 3M have developed some awesome products to repair plastic. There are a few folks out there who may prefer to use a plastic welding method, but the adhesive products have really made this a simple process. Sure, there are different concerns for repairing plastics than repairing metal, but when plastic is repaired properly, it’s as good as new.
One Sided Plastic Repair Using Adhesives
There are times that a one sided repairs may be appropriate when making plastic repairs. If the damage does not protrude through the plastic part, this may be an option. This will also save time because you will not need to remove the part, which will save time.
When performing a one sided repair, the front side will need to be properly prepared. As with any type of auto body repair, first wash the surface with dish soap and water. To assure all of the contaminates are removed use a cleaner to wipe the surface to be repaired. However, it is important that you only use the cleaner before the repairs are started. After this time, there should not be any solvent cleaners applied to the surface. Using solvent cleaner on raw plastic will cause problems.
Once cleaned, dish out the damaged area using a grinder with 50 grit sandpaper. Once dished out using 50 grit, use a dual action sander with 80 grit sandpaper to further extend the repair area. This will continue to dish out the repair area while providing a larger repair area to apply the repair adhesive. The last step is to feather the edges with a dual action sander using 180 grit sandpaper.
Once the repair area is dished out and feather edged, the surface should have a smooth dished out area without no rough or hard edges. Using compressed air, blow the repair area clean. Remember, do not use a cleaner, just blow clean using air. Before applying repair adhesive, adhesion promoter should be applied to provide adequate adhesion. Only spray a light coat, as the adhesion promoter should not be applied wet. If applied too wet, this can lead to problems with the adhesive adhering properly. Allow the light coat to dry for at least five minutes. However, if the part has been spray with adhesion promoter and left to set longer than 24 hours, another coat must be applied.
Once the adhesion promoter has allowed to dry at least 5 minutes, but less than 24 hours, the adhesive can be applied.
3M and many other brands of plastic adhesive repairs come in tubes, which mix the two parts in the nozzle. However, purge the tube before attaching the nozzle to assure both parts are squeezing out of the cartridge properly. Then attach the nozzle and squeeze a little adhesive through the mixing tube to assure the adhesive is mixing and changing color when is comes out of the nozzle. If some of the adhesive product is not mixed, it will not cure properly.
The rest of the process is much like body filler. Apply a tight coat to push the adhesive into the scratches to eliminate any air pockets. Next apply more adhesive to build the area up to fill in the dished out area.
Allow the repair to dry and sand the adhesive filler using 80 grit sandpaper and a block to block by hand. You can also use 80 grit on a dual action sander, but this will not level as well as blocking by hand. Once the surface is shaped, apply guide coat and reblock the area using 180 grit sandpaper.
If the guide coat sanded off and the repair looks good primer can be applied. However, before applying primer, adhesion promoter must be reapplied. If fact, any bare plastic should be sprayed with adhesion promoter before applying coatings. If the repair still remains low, an additional coat following the same steps, including the adhesion promoter can be followed to attain the proper contour of the part being repaired.
Two Sided Repair Using Adhesives
If the plastic part has a crack all the way through the part, it will need a two sided repair. This may also repair the part to be removed from the vehicle to gain access to the backside of the part. Once the part is removed, the part will need to be washed with soap and water as with any auto body repair. You will need to properly clean the inside as well to remove all dirt, wax, grease, etc. Once cleaned, use compressed air can be used to dry the part.
Many time when the damage was bad enough to cause a tear or rip, the plastic part may have an irregular shape. If the part is theroplastic, such as many bumper covers, the part can be easily reshaped using heat and quenching the part. Once shaped correctly, the backside of the part must be ground using a 50 grit sanding disc on a 3 inch grinder. Grind several inches past the damaged area to assure the adhesive will adhere properly to the plastic. Now flip the cover over and dish out the front side of the part as described in above for the one sided repair. the one sided repair method. The part may need to be clamped in place with making the backside repair. It is also a good idea to place a piece of tape on the front side to prevent adhesive from falling through when adding the adhesive to the backside.
Flip the part to where you can access the backside of the part. Blow the area to clean it with compressed air. Before applying the adhesive, spray a thin coat of plastic adhesion promoter to the repair area of the plastic to assure proper adhesion and wait about 5 minutes to apply the adhesion promoter to flash. While waiting of the adhesion promoter to flash, use a pair of scissors to cut a piece of fiber reinforcement that extends past the repair area about one inch. After the flash time apply a coat of plastic repair adhesive to the back side of the part. Spread the material using a body filler spreader or a stiff brush. Next, place the fiber reinforcement on top of the plastic adhesive. Add another coat of plastic adhesive on top of the fiber reinforcement to assure it is completely covered. Now allow the repair to dry the recommended time. Once dry, flip the part over and remove clamps and tape. Now a front sided repair can be made as described for one sided repairs.
Repairing Backside Using A Patch
I mentioned using adhesive and reinforcement to repair the backside. However, 3M also makes a patch that can be used on the backside. The frontside will follow the same procedures. When preparing the backside, do not grind or sand. Simple clean the surface with soap and water. If there is any paint overspray on the surface, you can use a red scuff pad to remove the paint overspray. Then clean and use the adhesion promoter pad that is included the patches. Allow the adhesion to dry for 5 to 10 minutes. Cut the patch to extend over the damage several inches. Remove the backing off of the patch then stick the patch to the backside of the plastic repair area. Now the part is ready to repair the frontside.
Using The Magna-Stitcher
Using The Magna-Stitcher 2500
Motorguard recently came out with a new tools for repairing the back side as well. Again, prepare the front side and with the other repairs. Then attach the Magna-Stitcher to a stud welder gun. If using the Magna-Stitcher 2500, a stud welder gun will not be needed. Next, use a stake to apply to the backside. This melts the stake into the plastic holding it together. While the plastic is still hot from the stake, give it a little twist to lock the stake in place. Allow to cool for a few seconds before removing the tool. This will allow the plastic to harden and hold the stake in place.
Apply a staple about every 1/2 inch to 1 inches apart. Once this is complete, the part should be supported with the stakes. Using wire cutter, cut the ends of the staples left sticking out. Now you’re ready to start the frontside repairs. Simply follow the front side repair methods described above.
Tips For Grinding and Sanding Plastic
If plastic contain polyofins, the plastic will smear and melt easy when sanded and ground. To reduce this, use low RPM when using a grinder or DA sander. If there are any smears or balled up plastic after sanded, be sure to remove them before continuing repairs.
Adhesion Is Key When Repairing Plastic
The biggest problem with flexible plastics is adhesions. That is why the adhesion promoter must used and must not be skipped during any of the repair process.
Plastic Tab Repair
Tab Repair Demonstration
A common problem on bumper covers is that the tabs that attach the bumper to the car get broken. This is critical, as this is what secures and aligns the bumper cover. Many times, a broken tab is all that is wrong with the bumper cover. With the cost of these plastic parts $200 to $700, the replacement cost can break the bank.
Depending on location of the tab, the bumper may need to be removed. However, many times this repair can be made with the bumper cover still attached to the car.
Start the repairs with a good wash to remove contaminates on the bumper cover. This will assure all silicones are removed which will provide a quality repair.
Where the tab broke, use a 50 grit roloc disc to grind the repair area to a point. This will provide the correct shape to maximize adhesion. When grinding plastic, use a slow RPM.
Using 80 grit by hand or 80 grit on a dual action sander, sand the repair area to assure all areas the adhesive will come in contact with is sanded. Do not apply adhesive on an un-sanded surface.
Drill holes on repair location using a 1/8 inch drill bit. This will allow the adhesive to flow from one side of the repair to the other side providing a rivet like effect.
Using compressed air, blow the repair area clean. Do not use any type of solvent cleaner, as this can absorb into the plastic, which will cause problems.
Spray a light coat of adhesion promoter to the repair area. This is important so the adhesive adheres to the repair. Allow the adhesion promoter to dry for 5 to 10 minutes.
Cut a piece of plastic from the package the tab repair adhesive came in to shape the adhesive. Support the bottom of the plastic using a spreader. Apply adhesive to the plastic, then lightly press the adhesive to the backside of the repair. Next, wrap the plastic around the tab location. Make sure the plastic extends past the original length of the plastic tab. Lastly, lightly apply pressure using a spreader on the top of the adhesive to shape the tab. Do not apply too much pressure, as this will build a tab that is too thin. Allow to set-up 30 seconds to one minute. The release pressure on spreaders and allow to finish drying.
After a minute or two, before completely drying, precut the tab a little larger than the tab size. Then allow to finish drying about 5 minutes.
Once the tab is fully hardened (about 5 minutes) remove plastic from around the tab and shape the tab using the 50 grit roloc disc, followed by 80 grit by hand or on a dual action sander.
The last step is to drill the hole for the clip or bolt to go through to attach it to the vehicle.
Warning – You only have about 30 seconds work time with this tab repair adhesive. So be sure you have everything you need before applying the adhesive to tab location. Be sure to watch the video for additional information and how to purge the adhesive cartridge before using.
SMC – Fiberglass
The main raw material used to make fiberglass is silca sand. Silica sand is also the main raw material used to make glass. The fiberglass fibers are made into a yearn like material. When fiberglass is mixed with a resin, it becomes very stiff and hard….hard to sand too!
Fiberglass worked well for making parts, as it can easily be molded to fit different shapes. Although this type of fiberglass is not generally used on modern day car parts. Most car parts are using a similar composite called (SMC) sheet molded compound.
Fiberglass Auto Parts
There are still a lot of parts made from fiberglass today. However, most of the automotive parts are made from SMC, which we will discuss later in this guide. Although, some aftermarket parts, such as body kits and spoilers are still made from fiberglass. If one of these fiberglass parts are damaged, the traditional method covered in this section can be used to made the repair. Other things made from fiberglass include boats and jet skis. I am not up-to-date on modern day boats, but the last I know of, fiberglass is still being used for manufacturing boats.
WARNING ABOUT FIBERGLASS
Before we start the repair process, I would like to caution you of the health hazords when working with fiberglass. When fiberglass is sanded, the fiberglass dust becomes airborne, which can end up in your lungs. Proper personal protective equipment (PPE) should alway be worn when sanding. Fiberglass brings additional things to consider. If you breath fiberglass particles in, it can cause health problems. If the fiberglass particles get on your skin, it can irritate your skin and cause you to itch or may even cause a rash. If you ever worked around fiberglass, then you know what I am talking about.
MY First Experience With Fiberglass Itch
When I was in high school, I worked at T-Bird Home Centers, which was a was a lumber store. T-Bird lumber store was not like Home Depot, as much of the inventory, such as lumber, pvc piping, siding, and insulation was stored outside under covered areas. My job was to unload the items that were going to be stored outside under the covered sheds and stock them. When someone bought one of these items inside the store, they would announced over the intercom and I would be out there to load the items for the customer.
Anyway, one of those items was the fiberglass insulation. I still remember wearing a jacket on cool morning and taking the jacket off as the day got hotter. What I really remember is grabbing my jacket and putting it back on after stocking or load a lot of fiberglass insulation. That has to be the worse feeling ever. The feeling of something rubbing up against your skin pushing the fiberglass further down into your skin. It is a feeling you will never forget. If you have ever worked with fiberglass you know exactly what I am talking about. If not, you will if you ever work around fiberglass.
As irritating as this is to your skin, can you imagine what is does to your insides. I know I got a little carried away making this point, but I want to make it clear, wear proper PPE when working with or sanding fiberglass.
Fiberglass repairs follow a similar process as plastic repair. If the damage is cracked or broken a two sided repair will needed to be made. However, if the damage is only on one side of the part, a one sided repair can be made.
One Sided Repairs
As with plastic, the damage will need to be dished out to allow fiberglass layers to be applied to repair the damage. Using 36 grit sandpaper on a grinder grind the damaged area. To properly dish the fiberglass, sand deep into the damaged area and taper the fiberglass out until it feathers into the outside of the undamaged fiberglass. To help taper into the undamaged fiberglass a dual action sander can be used with 36 to 80 grit sandpaper. Once the the repair area has been ground, dished, and feathered, you are ready to begin the repairs. Depending on how deep the dish is will determine how many layers of fiberglass will need to be applied. I usually try three layers to begin with. Additional layers can always be applied later if needed.
Preparing Fiberglass Matt
First, cut a piece of fiberglass mat to fit on the deepest part of the repair area. Then I cut a piece slightly bigger, which will start filling in the tapered area. Then the third piece of mat is cut to cover all of the repair area that was tapered.
Applying Fiberglass Layers
Once all fiberglass is precut, I put on gloves and start mixing the resin and hardener. Using an ounce mixing cup, I pour the amount I think that I will need to apply all three layers. Read on the can for the specific resin you are using for mixing ration. For instance it may say to add 12 drops of the liquid hardener for every ounce of resin. If you poured 3 ounces of resin, you would add 36 drops of hardener. Then mix the resin using a paint stick to mix the two products together.
Next, using a cheap through away paint brush, dip the brush in the resin and spread over the repair area. Then apply the first, smallest piece of fiberglass you precut and apply it to the repair area. Once applied, brush another layer of resin over the fiberglass. It is important to completely saturate each layer of fiberglass with the resin. Once the second coat of resin has been brushed on, apply the second layer of fiberglass that is slightly larger than the first piece. After applying, brush another coat of resin over the fiberglass. Then apply your third and largest piece of precut fiberglass that covers the entire dished out repair area. Then apply the last coat of resin over the fiberglass.
Once the last coat of resin has been brushed on, use a fiberglass wheel or body filler mixing spreader to push down on the fiberglass to assure all air bubbles are pushed out of the fiberglass layers.
If there is any resin that ran or dripped, it can be cleaned immediately using laquer thinner. If it is allowed to dry, it will become very hard and will have to be sanded off.
Sanding The Fiberglass
Once the fiberglass has dried it can be block sanded to shape the fiberglass back to the panel shape. If the repair area is still low, sand the low area with 36 grit and apply another one or two layers to the low areas to build it up to the desired height, allow to dry, and re-sand.
Minor Lows and Imperfections
If there are only pin holes, minor lows, or other minor imperfections, a coat of body filler can be applied over the fiberglass to fill the imperfections. Then block sand the body filler. Once the repair has been shaped using 36 grit sandpaper, minimize the 36 grit scratched by sanding with 80 grit followed by 150 grit to prepare the repair area for primer.
Completing The Fiberglass Repair
Once the fiberglass has been finished out using 150 grit sandpaper, you can prime and block sand using the same method as with other repairs. However, there is no need to use epoxy primer for corrosion protection or adhesion promoter. Simple clean the repair area and spray primer surfacer to help fill scratches and any remaining imperfections. I have a book “How To Prep a Car Paint Finish” that will take you step-by-step through the priming and blocking stages of the repair.
Once the repair has been primed and blocked, you can paint it as you would any other automotive part. Different paint companies have different ways, but I use a primer sealer followed by base coat and clear coat.
The Difference Between Fiberglass and Sheet Molded Compound
Fiberglass is a mixture of fiberglass and resin and sheet molded compound (SMC) is a fiber reinforced plastic. They look similar, but use different repair materials to make the repair. SMC must be repaired using an adhesive designed to repair SMC. Regular fiberglass abd resin will not adhere properly if used to repair SMC.
How To Tell The Difference Between Fiberglass and SMC
The easiest way to determine if the part is fiberglass or SMC is to look at the backside of the part. If you can see the fiberglass stands on the backside, then it’s fiberglass. If the part appears smooth and without texture, then it’s SMC. However, if either part is broken into, fiberglass strands will proceed out of the edges of the break. This is what could be confusing if not observed a little closer before determining if the part is fiberglass or SMC.
Lesson 6 Resources
- 3M Flexible Parts Repair
- Reinforcement Backing Material
- Adhesion Promoter
- www.CollisionBlast.com/3MDA – DA Sander
- 80 Grit DA Sandpaper
- 180 Grit DA Sandpaper
Product Technical Sheet
Watch the videos and read the technical data sheets. Once you can completed the content, visit a body shop to ask for a scrap bumper cover if you don’t already have one. I would perform a practice repair on a part that does not need to be used for a car. Plastic repair is fairly easy and you should have the hang of it after several repairs.
Plastic Repair Using Adhesive
- Wash plastic part with soap and water.
- Clean part with a plastic cleaner to assure all contaminates are removed.
- Using a 3” grinder with 50 grit sandpaper, grind several inches past the damaged area.
- Using a 3” grinder with 50 grit sandpaper, dish the front side of the damaged part. Once damaged is dished out, grind an additional two to three inches past the damaged area. As with metal, you do not want adhesive applied over paint.
- Using a DA sander with 80 grit, sand the edges of the front of repair area, then follow with 180 grit sandpaper to feather edge the repair on the front side.
- Blow front and back with compressed air to clean surface.
- Apply 1 – 2 thin coats of adhesion promoter. Allow to dry 5 minutes or recommended time as stated by brand.
- Apply tape to the front side to prevent adhesive from dripping out.
- Apply a tight coat of adhesive to the back side.
- Cut and apply reinforced fiber and apply another coat of adhesive on the top.
- Allow to set up.
- Remove tape and apply adhesive on the front side, a tight coat followed by a fill coat.
- Allow to dry and sand with block or DA with 80 grit sandpaper.
- Apply Guide Coat and sand with 180 grit to remove all 80 grit scratches.
- If there are still lows this process may need to be repeated, however, if it is a small imperfection or pinhole, flexible parts putty may be applied and sanded.
Plastic Tab Repair
- Wash With Soap and Water
- Grind and Sand Tab Repair Area To Point
- Drill 1/8 inch holes in repair area
- Clean repair area using compressed air.
- Apply thin coat of adhesion promoter to repair area and allow to dry 5 to 10 minutes.
- Cut Plastic and Apply Adhesive to repair area
- Precut Tab
- Shape tab to the original shape
- Drill hole where tab originally had the hole.
Module 6 Plastic Repair Quiz
Once you have completed the module content, take the module 6 quiz below. Please note that you must score 75% or higher to receive a certificate. This quiz is short because you have the 3 additional quizzes above There are only 8 questions. There is not a time limit set on this quiz so take your time. Find the correct answer if you do not know for sure. The objective of this quiz is to help you learn the information if you do not know the answer, not guessing. Good Luck!