Epoxy Primer: Does superior adhesion require it to be sanded before applying base?

That's the question. Is it necessary to sand epoxy primer in order to have best base coat adhesion? I'll shoot some test panels tomorrow to see what they indicate but what do the experienced painters say? Rocks have chipped the paint (the clear and base but not the primer) on some of my bikes; the failure is always at the base/primer interface, which I've never sanded in the past.

Yes, sanding is required.

Keep in mind, these finishes are designed to fail by chipping to the primer layer, preserving the substrate beneath. The thickness of base and clear layers have greater impact on chip resistance than mechanical adhesion with the primer.

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Originally Posted by Rody

Keep in mind, these finishes are designed to fail by chipping to the primer layer, preserving the substrate beneath. The thickness of base and clear layers have greater impact on chip resistance than mechanical adhesion with the primer.

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Now THAT is interesting. Thank you Rody!

I often think of "what part do I want to fail first" in a mechanical/machinery sense but never extended the concept to paint! ...and my paint failures are, by design or luck, consistent with that.

So you sand EVERYTHING? All the little diamonds and discontinuities? That is tedious! How do you even do it??

Photos of my cobbled together, supplied air respirator, "blast cab" and paint "booth" start here for anyone who's interested: Flickr

It turned out pretty well and is in the "drying oven", aka shop attic which will hit 130F for 3 or 4 hours/day this time of year.

Yes, every thing is given a mechanical tooth.

I use 600-800 grit paper, depending on the build layers, dry sanding.

I usually employ a variety of implements to get the little, fiddly areas. You don't have to go crazy, but do need to break the surface tension uniformly.

Blow off with filtered air, wipe with a tack cloth, then ready for base layers.

r

Quote:

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Originally Posted by Rody

Yes, every thing is given a mechanical tooth.

I use 600-800 grit paper, depending on the build layers, dry sanding.

I usually employ a variety of implements to get the little, fiddly areas. You don't have to go crazy, but do need to break the surface tension uniformly.

Blow off with filtered air, wipe with a tack cloth, then ready for base layers.

r

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Thank you!

That is one of those little details that is incredibly useful.

John, I usually follow epoxy primer (PPG "DP") with a sandable primer to fill in the irregularities and zits, also any flaws/runs/bug LZs in the epoxy. That gets wet sanded, usually with 220 grit. DP stays kind of gooey for a week or so and clogs up wet-or-dry pretty fast. When it is in its gooey state it has excellent adhesion to whatever goes on top. I usually give it a day before putting the sandable on, then seal the sandable with another light coat of DP. And shoot a base coat over that within a day.

That gives me a pretty durable paint job, especially with PPG DCC or DCU, or that orphan can of aircraft-grade Imron I had ...

jn

I'm similar to Jon in that I use PPG DP epoxy primers. With new frames, I generally don't need to sand before the first base coats because I'm not dealing with dents, rust pits or other manufacturers' defects. However, if I can't get the first base coat onto the DP within about half an hour of shooting it, I will then shoot another very reduced coat of the DP before the base coat. For the most part, I'm using Imron "EU" base coats and they stick to non-cured DP amazingly well. And once that first coat of urethane is on there, the rest (other base coats or clears) stick at least as well as they burn in anyway. I figured this process out many years ago simply as a way to save time and effort. Even though I have to do a lot of sanding during the process, If I can eliminate some of it with the same or better results, I'll do it. Hope this helps.

Thank you guys for the tips. It's a big help!

I just finished the last frame I'll ever paint.

Lacking a blast cabinet and spray booth makes frame painting a serious PITA. I cobbled together a supplied air respirator that works well for painting and blasting; it does what it needs to though the lens is a little bit rough from sand impacts. I blast in a large trash can in the carport so I can recover the media but it makes a hell of mess. I also paint in the carport; my shop exhauster isn't spark resistant and can't be easily made so; too many brushes for grounding it's components would be required so making a "booth" of my shop wasn't feasible. Setup of my arrangement is very slow and an enormous pain. I could manage to put up with it for my personal frames but not frames for friends and family, but what I can no longer tolerate is the pollution it generates. It's a completely uncontrolled, point discharge of an awful chemical aerosol. So, I am finished painting. Forever. I just can't do it any more.