Etching primer vs epoxy primer

Some prefer etch and some prefer epoxy. DuPont has historically pushed the etch primer and PPG has historically pushed the epoxy. I much prefer the epoxy for reasons previously posted.

I always let the epoxy dry (to the touch) before topcoating. In fact PPG's high end epoxy, DPLF has a 60 minute minimum wait time (70 deg F) before topcoating (90 min if 2 epoxy coats are used). That's with the standard activator (catalyst). Plus if you are going to put any plastic filler on top of it (I sometimes do) it needs an overnight dry beforehand (again if 2 epoxy coats are used, which is typical for me). The DPLF can be topcoated up to 7 days after application without scuff sanding and reapplying. I am not saying other methods can't work. I just prefer to follow the explicit manufacturer's instructions.

Because of the high price for DPLF I have moved to PPG's economy line MP170 epoxy. The instructions are different. The initial dry is 30 min at 70F (1 coat) and the topcoat window is 3 days instead of 7.

DuPont's epoxy instructions are different still with a 30 min dry for 1 coat, a 16 hr dry under plastic filler and a maximum 24 hr window to topcoat w/o sanding. The 24 hr window is too tight for my liking. I will therefore be unlikely to ever use the DuPont product.

Whichever product you chose, etch or epoxy, be sure to obtain the manufacturer's technical data sheet (not just can label instructions) that details usage and compatibility for the particular product.

Rod

The orginal purpose of self-etching primer was to condition new metal in order to allow regular laquer primer to stick better. Many painters are no longer using the SE primers because of the excellent adhesion properties of the urethane and epoxy primers.

If you use SE primer then you need to wait several hours before spraying the primer and the top coats. If you spray too soon the acid in the SE primer has not evaporated and that will cause peeling later as the acids leaves and passes through them. My mentor told me to wait a few hours and then scratch the surface of the SE primer with a Scotch pad and let it set for several more hours in order to allow complete drying.

As far as top coating the SE primer, it is considered best to use a urethane primer first and then the paint. Rarely do you paint directly over the SE primer. I have done it, but only in areas that are not exposed directly to the elements.

When considering epoxy primers keep in mind that if the epoxy dries before the top coats are sprayed, then you are going to get peeling. Look at all the GM and Chrysler cars whose paint is literally falling off, especially the whites and the high metalic blues and grayes.

If you choose epoxy primer then everything must go smoothly with no loss of time. In other words the paint goes on before the epoxy even has a chance to dry. I have never used a drying retarder in E prime to prolong the wet time, but it might be an option.

Should the epoxy primer dry then you will have to sand in order to get good adhesion. But here is the trouble: E primer does not sand well. It gums up the paper quickly and leaves a rather rough, uneven surface behind, not the smooth textured surface that urethane primer has. Imagine trying to sand a warm candy bar and you have an idea of what it is like trying to sand E prime. E prime that is several years old sands much better, but fresh E prime is a tough go.

Each has it's own unique characteristics and purpose. Epoxy is used for strictly adhesion properties before final painting. Epoxy is sprayed on and the paint is applied before drying and without sanding. Urethane is used as a primer or surfacer for working the metal into shape. It too can be painted over just prior to it's drying or can be allowed to dry completely and then sanded and painted.