What’s better, high build primer or body filler?

Question:

I have read several articles regarding body surface preparation. Some articles refer to “high build primers” and others refer to “skim coating” using body filler. Both materials are applied to the body surface in preparation for block sanding. Which material is better from the standpoint of adhesion? It would seem to me the high-build primer would be preferred and would be the only surface material required if applied and sanded correctly. I don’t recall but I’m thinking the body filler would be applied over a primer.

Answer:

High build primers and body filler generally serve the same purpose and as long as the surface is prepared correctly and the materials are applied correctly you should not have any adhesion problems with either one.

In general, I’d use body filler in localized areas to address more serious surface deficiencies but not on the entire vehicle. It is very difficult to spread an extremely thin and even layer of filler over the entire body so unless you are very experienced you’re more likely to make a bit of a mess that wastes lots of filler and creates a lot of extra work. It’s much easier, on the other hand, to apply a very thin, even layer of high build primer across the entire body.

While it is customary to spray primer over body filler, the application of high build primer alone doesn’t mean this is the only surface material required. Depending on which specific manufacturer’s products you use and which line you go with, you’ll probably need to spray a sealer over the primer to give the paint the best surface to adhere to, a uniform underlying color to help avoid subtle color variations in the top coat, and to help hold down all underlying bodywork.

Whether you should spray on primer before applying body filler depends on what the car’s body panel is made from. With most metals, including steel and aluminum, it’s advisable to spray a self-etching primer over the bare surface to promote adhesion and discourage corrosion. With fiberglass, a self-etching primer is not needed.