Top coat adhesion to Epoxy primer?

B

Brad J.

Oldtimer
I spot sprayed epoxy on the bottom of the rear bumper cover where the stone chips were on my GTO last spring and then I base cleared the whole thing. Basically last spring I sanded it with 400 grit and did some epoxy work on the bottom. Then it was was shot with standox base and Euro clear last spring. The car has sat in pieces all summer, the bumper fell off my cart last week and did enough damage I decided to sandblast it last weekend to strip it bare. I was very surprised that the base clear blew off the epoxy very easy and held much tighter to the sanded clearcoat. I'm sure it never would have been a problem but I am very surprised as I thought the adhesion would at least match the sanded clearcoat. I was wondering if anyone else has witnessed this as I thought using epoxy as sealer has the best topcoat adhesion.
 
Do you recall how much time elapsed between the epoxy and the basecoat? Did you in fact use the epoxy as a wet on wet sealer, or as a primer? I'm curious, because I use a lot of epoxy, too.
 
I used the epoxy for a sealer but I did put two coats on bottom. I let it sit overnight like I always do when I use two coats and sprayed it the next day. I never sanded any of the epoxy. The epoxy was stuck to the plastic great, it's just the top coat came off pretty easy with the blaster.
 
I have no insight into this problem, but I have a lot of stuff running around out there with similar materials and techniques.

So I am a little nervous about this.

In a perhaps unrelated but similar vein, I have noticed poor adhesion of filler on unsanded epoxy after about 3-4 days, so I have been scuffing the primer to apply filler after the second day...
 
I'd like to do a few sample experiments to check adhesion of wet on wet vs. sanded epoxy. After thinking about it, I wish I would have stopped blasting and checked it with a putty knife. I do know that it was getting cold out and I only blasted the bottom of the cover instead of the whole thing because the base/clear that was on the sanded clear was very slow to remove. I'm sure I had enough adhesion on the lower part of the cover that it wouldn't have been a problem. I was under the understanding that the epoxy was one of the best sealers for adhesion available and to see for my own eyes the adhesion was a lot better on sanded clear makes me question it. I think I have some rocker corners for the same car that were epoxied at the same time and then painted. Maybe tomorrow I will sacrifice one and check for adhesion.
 
I have not had a lot of time lately for testing, If you decide to check those out I will be reading with great interest!
 
I drug the bumper cover out of storage this morning and gave it a look over. Since I was lazy and cold when I was blasting I gave up on it. I didn't blast the entire bottom so I still had some areas with epoxy and topcoat. I grabbed a razor blade to compare the adhesion between the sealed/topcoat and sanded/topcoat. Note: The areas in question had a nice blast line in each so I could easilly get a blade in there to flake each. They flaked about the same and by no means could run the blade down like laquer job. Just small flakes would chip off each surface. One thing I noticed with the epoxy/topcoat side is the backside of the yellow base/clear had gray epoxy on each chip. So really the base adhered to the epoxy, it's just like the epoxy itself sheared off. The sanded/topcoat chips wouldn't completely flake away clean either. Some of the base was still adhered to the sanded original. Possibly with the blaster the sanded topcoat just withstood blasting a little better than the epoxy? I'm not so worried about it now. It has been on my mind the last couple days as I have a lot of jobs with this same treatment.
 
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