Using Guide Coat
Guide coat is a coating with a contrasting color to help identify imperfection such as highs, lows, or scratches. I recommend to always use guide coat when block sanding primer surfacer. It can actually reduce the amount of block sanding and I don’t think anyone will complain about that. How this can reduce the amount of block sanding is because once all of the guide coat is sanded off, it’s good…and you can move on. Without guide coat you may find yourself over sanding to assure it is blocked properly. So don’t think you don’t need to use it…use it and make your life easier.
On this Mustang project we used Eastwood guide coat that comes in a spray can. They have several colors to choose from. Select the color that will contrast or stand out from the primer surfacer used. Guide coat also comes in a dry powder form that is rubbed on using a foam applicator. Both work well so it is just a preference to which you prefer.
Don’t use a cheap spray paint. It may cost less, but this will gum or clog up the sandpaper causing more effort, take longer, and go through more sandpaper.
Sandpaper Grit Used For Block Sanding Primer
I like to use 220 to 320 grit sandpaper on a block. If I am pretty sure that I will need to prime and block a second time, then I use 220 to cut better and sand faster. However, if my body work was awesome and I know it is only going to take one prime and block, then I will use 320 grit. Once blocked with 320, then I can give it a final sand using 500 grit and now we are ready for paint.
We blocked the Mustang with 320 because we felt good about removing most small imperfections before priming. Now after blocking, if you do have a spot or two that did not block out, then you only need to prime and block those areas. There is no need to re-prime all primed areas again and block sand everything again.
Cross Sanding and Keeping Block Flat
I talked about sanding off the guide coat, but keep in mind that getting the guide coat off is not the objective. The objective is to level the surface and the guide coat helps us see any spots that do not level out. So whatever you do, don’t get obsessed with sanding the guide coat off and start tipping your block on the edge or using a finger tip to remove the guide coat. That will remove the guide coat, but it will eliminate the whole reason you are block sanding, which is to level. If there is an imperfection left in the primer after block sanding, it will need to primed again and block sanded. To level you must keep your block flat on the surface and cross sand in different directions.
Shaping Body Lines with Masking Tape
If you have body lines that you need to be sharpen you can use masking tape to mask one edge of the body line off, then block sand the other side. Then flip it around and mask the other side with masking tape and block sand the opposite side. We only use this in areas where we had body work on body line edges. However, some people who build show cars use this technique on every body line on the car. This just helps create sharp straight lines. This same technique can be used with body filler and glaze putty if needed.
Block Sanding Concave Areas
If cars were flat, and then flat blocks would work for the whole car, but cars have every shape imaginable. Concave shapes can be difficult, this is when the panel curves inward. This shape is going to hit harder on the edges of a flat sanding block and leave sanding lines that will be seen after painting. For these areas we need to find a block that fits better. There are half round and fully round blocks that works much better. Motor Guard and other companies make blocks you can use, but in the past I have used pieces of broom handles and PVC pipe as a block. That will work, but I do like the round blocks made for this much better. There are many different shapes and sizes of sanding blocks. Select the one that fits best and remember, the bigger the block, the better it is going to level the primer.
Wet or Dry
This is a question I am asked often so I thought I would throw it in here. On the Mustang and all our project we use a dry sanding method. However, for many years in my career I blocked sanded primer using a wet sanding method. I think most body shops have started using the dry sanding methods to eliminate the amount of sanded primer that get washed into their drain. While dry sanding eliminate that issue, it does produce more dust. So there are pros and cons to either sanding method. So this is going to be a preference of the person block sanding. The same is true for final sanding. You can wet sand the car with 400 to 600 and get do a great job. Although, it is going to take more time and effort than using the dry DA sanding method.
To get back to the steps we took on the Mustang project, we block sanded the primer with 320 grit dry. But like they say, “there’s more than one way to skin a cat.” Now I,m not sure why you would skin a cat, but that is the saying just meaning there is more than one way to do something.
After block sanding, there were a few small spots we sanded through so we reprimed those areas and applied primer again and block sanded those areas again using 320 grit sandpaper. Remember, it sanded through to the metal, self etch or epoxy primer will need to be applied before the primer surfacer.
Now the Mustang is block sanded with 320 grit. Next, we sanded the primer areas with 500 grit to minimize the 320 grit scratches. We’re getting close to the paint booth….we’ll be spraying paint soon.