Hey guys, I'm new to the forum. I'm working on respraying an aftermarket bumper. It came with a primer on it when I bought it years ago. At the time I had to no idea what this primer was so I just scuffed it and sprayed my new primer, which is a 2k urethane.

Long after the primer had dried, I started to noticed it lifting in certain areas, mainly on tight inside corners. Upon removing the bubble that was formed, I noticed the primer underneath now had a textured surface. This gave me the impression that it was getting dissolved by the urethane I was spraying on top. If I wipe the original primer with a rag and some lacquer thinner or acetone, the rag will take some primer off with it, my guess was that its a lacquer after seeing this.

I was under the impression that urethane sticks to lacquer, assuming that I am correct about it being a lacquer. I tried stripping it off the bumper so I could just work with bare fiberglass, but it apparently sticks to the fiberglass very well. My pressure washer wont lift it, sand paper just gets caked up, haven't had much success with chemical strippers either. This bumper has a many ducts and tight corners, so it makes prep work extremely challenging.

Is there something I can spray on top this crap to guarantee my urethane will stick to it?

I'm a little confused by your post, but it sounds like where you sprayed a urethane top coat the lacquer primer beneath started to fail? And you are wondering now if the remaining lacquer primer on the bumper will be ok if you shoot a sealer or something over it?

I've heard of guys with ancient lacquer finishes on old cars sealing them with an epoxy sealer but honestly I would never take the chance. Generally the only thing you can shoot over lacquer is more lacquer.

That cheap primer should come off with lacquer thinner. Once it's all off clean the bumper real well, scuff it with a maroon scotch pad and shoot adhesion promoter and a sealer over it. Then you should be good to go.

So assuming I can't entirely get it all the off, what would you recommend I seal it with, an epoxy?

If it was just lacquer the removal would be much simpler, but there are several coats of urethane on top, and only about 50% of it wanted to come off. So I'm working with this patchy mess, I figured I would just address what the pressure washer was able to lift, and seal over the rest.

I already have the urethane base coat and clear, so I'm committed to spraying them on top of whatever primer I seal it with.

Epoxy is best bet given what you have there.

I just don't understand, I remember when clear urethane first came out.
that was before basecoats, painters were using lacquer and clearing
the car with urethane, that was the newest and best way to do it.
Painters doing graphics loved it and it made life so much easier.
Now days everyone says that they're not compatible.
Either somethings changed or they're all nuts.

I'm sure you are right about that, Jim - on the clear coat specifically.

We've had a few people here over the years start a thread out with "I shot my paint over this black primer that wipes off with lacquer thinner...." and the rest of the question asks why the paint is failing.

So generally I tell people not to risk it.

I've seen several paint articles and even tech sheets for certain urethane paints claim urethane can be sprayed over lacquer.

Maybe these primers are not actually lacquers, but some other paint that wipes off with lacquer thinner?

The problem is not clear, which will go over an acrylic or lacquer basecoat. In fact, most basecoats are a modified acrylic anyway, not urethane.

The problem that OP is having is with the bond between the factory primer (probably acrylic) and his "2K urethane" primer. Note that the problem spots are inside corners and the like. These spots are notorious for not getting proper preparation so part of the problem may have been insufficient scuffing/sanding plus maybe insufficient cleaning or wiping off of Prepsol.

OP also says that when he peeled the paint off the primer underneath was wrinkled. The other problem, which we should all have experienced, is that it's difficult to get even coverage in these tight spots so we usually pile a bit more in there to be sure. All the primers I use have an amount of thinners in them. Sometimes basecoat reducer, sometimes 2K thinner but up to about 10% just to make them sprayable, or sometimes more if the intent is to just get a surfacer, rather than a high build. Combine this solvent with a heavier application of a 2K product and you'll get softening of the underlying primer and solvent entrapment once the 2K primer starts to go off. I've had it happen even on flat areas that I've tried to build too heavy with the first coat of 2K primer.

The problem gets 10x worse if you try to force dry or accelerate the 2K primer, causing the top layer to harden before the bottom.

Remember, too, that this is on a plastic bar, not a metal panel. So the surface on it was not the black coating sometimes called "e coat" but a proper primer.

The crazy thing is that if he'd just scuffed and cleaned the factory primer and gone straight on with his basecoat the problem probably wouldn't have happened.

that makes a lot of sense to me, the reducer in my primer softening the lacquer

so using epoxy primer that doesn't require a reducer is the safest bet for sealing over the lacquer

is there a jell coat on your fiberglass bumper? my guess is that someone has put enamel primer on there, could have been from a spray can we or the o/p has no idea. 2k primer should go over lacquer primer with NO problems. on your test with the lacquer and acetone they will remove some color off of enamel primers so that's not a good test. what you could do is puddle some lacquer thinner on a spot if its lacquer it may soften it a little if its enamel its going wrinkle it. this is what happened when you shot your 2k primer, it collects heavier on those inside corners and other pocket areas. it stays wet longer and starts reacting to the layers below. no mater what it is I sure wouldn't try covering it up your just asking for more trouble. have you tried air craft stripper or spray can bumper stripper. I would get some and strip it to the jell coat, it can't be that hard. might save you a lot of time and money
Jay D.