Primer: What To Shoot, When To Shoot, And What To Do With It.
Now shoot some primer...
Once you are done with your bodywork, you want to spray those areas with an epoxy primer.
Epoxys and etch primers are not made to provide fill or to be sanded. They're designed simply for protection and adhesion.
Primer surfacers (build primers) are made to fill and level as well as sand easily. They are not designed to provide adhesion to bare metal...
This is an area where many make the mistake of applying a primer surfacer (high build, sandable primer) directly to bare metal. There are cases where primer surfacer has peeled from bare metal.
So, epoxy primer first, primer surfacer over that.
The epoxy primer needs to "vent" solvents prior to spraying the primer surfacer. Let it dry completely. Then scuff the epoxy primer before applying the primer surfacer. Read the can for full directions.
Don't apply too much primer surfacer too fast as thick coats trap solvent and can cause shrinkage.
Remember, let the primer surfacer dry! I let mine dry a week or more. While this may not be necessary, I don't end up with waves in the paint. It lets the primer do any shrinking and become stable.
Start with a heavy-grit sand paper, such as 220 grit. The heavier grit will cut the surface level rather than follow the contour of the high and low spots. Re-prime with primer surfacer and sand with 320 or 400 grit to smooth it out.
Most, if not all, of the primer surfacer will get sanded off as the body is block sanded. Although primer surfacer is often referred to as high build primer, it should not be used as body filler. That leads to shrinking and waves. Do good bodywork.
A guide coat will show you if the panel is as flat and smooth as possible. What you want is a contrasting color. You don't need to cover the primer, just a mist coat is all that's needed.
If you remove all the mist coat fairly evenly as you're sanding, that's good. If you are leaving areas that aren't sanded, those areas are low. If you remove some areas all of a sudden, those areas are high.
When you break through to the epoxy primer, stop. STOP! Further sanding will only make the surface uneven again.
If you accidentally break through to the metal, don't worry, just touch up with epoxy primer...
If you have areas with guide coat still showing, you will need to hand scuff those low areas so the new primer will stick (like you did with the filler), clean, prime, and sand again. It often takes 2 to 3 times of priming and sanding to get a car real smooth. If it takes more than this, you need to do some more bodywork...
When you're done, it's on to the next article,
Return from Primer to Bodywork
Return from Primer to How-To-Build-Hotrods