I like SEM products and have never had a problem with them. I have used several different brands and I remember reading the directions that it may cause “softening” of body filler. However, a quick search and I found the following on Sherwin-Williams website:
Etch Primer Selection and Protection
The purpose of an etch primer is to provide rust and corrosion protection to the bare body of your vehicle.
Seems simple enough, right?
Well before you start applying any paint products, it is important to know whether you need to apply body filler to improve any imperfections. If that answer is yes, then nine times out of ten you will want to apply the body filler prior to using an etch primer. Many body fillers will not adhere to etch primers.
After the body filler application and necessary sanding is complete (or if it wasn’t needed to start with) you can begin applying an etch primer.
Etch primers (or sometimes referred to as wash primers) use a phosphoric acid to attach zinc to a bare metal substrate. The zinc within the coating provides a protective and sacrificial layer to moisture. Without the protective layer, moisture would penetrate into the bare metal and cause corrosion and rust. But with the etch primer, there is a barrier that will absorb the moisture and prevent damages.
In most circumstances a primer surfacer or sealer will be applied directly over the etch primer prior to a color topcoat.
We also have direct to metal (DTM) urethane and epoxy primers to choose from. DTM primers provide excellent adhesion to the properly prepared bare metal and can be directly topcoated.
Regardless if you choose to use an etch primer and then a sealer, or go straight to a DTM primer, proper surface preparation and film thickness must be met to ensure your vehicle has the protection it needs.
When you get to point where you are making paint purchase decisions, make sure to ALWAYS request the product data sheet. This easy to follow form will give you the instructions necessary to be successful!