Randryracer

DUB

It is a concern when you are dealing with the urethane bumpers where they generally have an elastrometric additive ( flex additive) added to the primer AND CLEAR COAT to make it flexible so it does not want to crack off in pieces if it gets hit.

The 2K primer that is sold by most major paint manufactures will do just fine. And the possibility of just applying sealer is also possible depending on how many layers you break through while trying to remove the clearcoat.

Use the primer from the company you plan on using the color of paint to paint the car. You do not have to do that...but sometimes it is wise to not switch brands of products unless you have tested it and see if it works or not.

DUB

Highflight

For the urethane bumpers, I've had very good luck with DP40 (Ditzler).
The main purpose of using it is to promote adhesion which it does very well. It's a two part epoxy primer, but it's not really a primer per se' as much as it is a great sealer.
Applying just a thin coat of it on a urethane cover will, first, give you a sort of spotty but uniform color that is easy to hide, and second, being a thin coat, it easily handles the flexibility of whatever elastomeric paint you follow up with.
It comes in other colors than the grey/green of DP40 with different numbers (DP**) to denote white or black or whatever.

I'm sure someone will come on with a bunch of instructional sheets that say not to use it on flexible parts, but long term, follow up experience over a couple of decades has proven it to be a good option if you remember to just apply one thin coat (I even reduce it a bit to make it thinner so that it's just a sealer and adhesion promoter, not a heavy body primer).

derekd8915

Highflight

If used correctly, you don't sand it at all. You can wipe it with a piece of 600 if you let it dry enough (overnight), but you lessen it's properties as an adhesion promoter.
It's best used for this purpose if you let it flash off for an hour or so, tack wipe it, and go paint.

You ask, so I'll get into more detail...
On a properly prepped bumper cover, you'll end up with w&d (wet&dry w/water) 600 grit sandpaper. You don't have to sand down to base material, but you do want it uniformly sanded to that grit.
If you spray the coat of DP40 over any remaining finish (which is ok of the remaining finish is mostly original and in great shape), you might notice what I would describe as "bleed through" where you can see the edges of original finish where it was sanded away.
That's ok, but if you do see that, let the DP40 dry a day and then lightly scuff sand it again with w&d 600 grit. Don't sand it off, just smooth it.
Then spray another reduced coat of DP40 on the cover, let dry an hour, tack off any dust that has settled, and then go right into base coat as I mentioned.
The trick of DP40 is to NOT let it thoroughly cure (it's a 2K) before painting. If you do let it cure, you MUST sand it again before respraying it with itself or with paint.

I again stress that you are not using it as a primer, but only as an adhesion promoting sealer that retains lots of flexibility (if you keep it thin as noted).
I never had a car come back to me in the 20 some years I had my Corvette shop* with a failure of cover paint, and some that came back to me that were hit on the cover surprised the hell out of me as to how well that process held up under collision stress.

*I opened my shop in 1972 and quit in 1995 to start a different business that I could actually make some real money at and retire comfortably (worked beyond my wildest dreams). Note those years. That meant that the ratio of the cars I worked on averaged throughout that time ended up to be about 5% C1's, 15%C2's, 55% C3's, and 20% C4's (all numbers pulled out of my *** from general memory). The seemingly low percentage of C4's I repaired/painted is because they were new enough during that time that most of them went back to GM dealers for repair. I usually saw them later to fix the butchering that the dealers bestowed upon them. The C2's starting in the late 70's started to take off in "collectability" so they were driven less and therefore wrecked less and needed paint less often. The C2's I saw from the late 70's on were mostly paint and restoration jobs, not so much collision.

DUB

It depends on what DP primer you use due to there are two different types if my memory of it serves me correctly. A DPLV and a DPLF. Some of the DP primers DO NOT like to be sanded on and will roll up or ball up if you get to aggressive while sanding. Back when the DP primer, first hit the market it was not designed to ne sanded on and more of a wet-on-wet system . Although it could be sanded on with care...it was not recommended. and the residue of water sanding would really **** off the EPA if they found out it got into ground water supplies.

I know from my experiences...the DPLF does not like to be sanded on....even light de-nibbing can be a hassle. I do not use the DPLV so I can not answer that but from what some other members have stated, it seems to be able to be sanded on.

It is some what comparable to an adhesion promoter but adhesion promoters generally...from my experiences are not applied on a panel where they are stacked on in layers. Adhesion promoters and etch primers ...for me ...are applied on in a manner that I may achieve a full covered coat but I do not try to stack them on each other too many times. Generally one coat and you are done and are complaint with how that product will do its job.

Even if the product is pigmented...it does not mean that the part you are spraying has to be covered completely to make that part the color of the primer you are applying.

Always verify how the product is designed to be applied by contacting the tech line for that specific product due to they are not all the same. Some may want it covered completely...some may not due to the intensity/strength of the chemicals being used in that product..

DUB

derekd8915

If this primer is allowed to cure for over 12 hours, but still within the window time, would a coat of sandable epoxy primer applied over it be ok? The reason I ask is I have some urethane bumpers on my c4 that need some minor repair, so spraying a sealer primer and painting right on top is not an option. I guess repairs could be done before the sealer, but I like to know all options before hand.

Highflight

Nope. A sandable epoxy primer is by definition a "building" primer, and that would promote cracking later on. You want to avoid anything sprayable that builds up to any thickness, even if it is supposed to be "flexible".
Any repair a cover needs should be repaired with an elastomeric product made for the purpose (of filling).
Once all done and sanded, then the DP40 is sprayed as a sealer and normal elastomeric painting ensues.

As DUB and I both said, DP40 is not intended to be sanded because it is designed as a SEALER, not a primer, and it was from it's introduction meant to be a wet-on-wet sealer (no sanding in between).
Yes, it CAN be sanded if you get any transition lines from underneath paint feathering you'd like to eradicate, but you have to let it sit overnight (24 hours), and any sanding has to be done wet, not dry because as DUB mentioned, it WILL ball up dry sandpaper and create more work. Don't do that more than once because you do not want to build up any more than you must.
Memory fails me so I don't remember exactly the product I used back then, but for cover repair, it was a two part filler that remained sort of rubbery when cured. It was specifically designed for urethane cover repair and was a bitch to get feathered properly, but it did work. It also could not be dry sanded, but only block/sponge sanded wet.

For DUB; to clarify, I am referring to the DPLF.

Randryracer

DUB

YES...That early formula of the DP primer was some NASTY STUFF.

AS for doing any work on urethane bumpers. I can say that they DO NOT LIKE any solvents being applied to them if you have exposed RAW urethane.

There are specially formulated products to wipe down a raw urethane bumper that do not cause the urethane to swell.

SO if you have a raw urethane bumper or areas that are exposed...for what it is worth. When I encounter this I clean those areas and immediately get it into the paint booth and get whatever I am spraying on those exposed areas applied. Allowing raw urethane to be allowed to be exposed to the environment AFTER it was cleaned prior to priming is NOT GOOD due to it likes to absorb those contaminants on out environment thus cause an surface contamination issue.

This can be easily seen when you what TV shows on how the Corvette was made and they show the urethane bumpers going down their own separate paint line adn the bumpers are cleaned and primed immediately and not allowing to linger around.

DUB