In one area of the car-rear steel quarter and plastic bumper below - i keep getting mostly horizontal cracks in my 2k primer a day or so later. Usually just one or two. And it's like it keeps moving on me! i sand and reprime or even reseal and re-prime and it cracks in a slightly different place. i do suspect a pervious repaint in this area of the car as the clear had cracked like a stained glass window. any ideas?

what i've done - its a respray. i sanded the car all over to rough it up, sanding this whole area more to get through the cracking clear. then after bodywork in some other areas i put down a seal coat of thinned epoxy in prep for immediate (that day) painting. i had some problem areas so didnt paint. during this time the epoxy got a horizontal crack or two. i sanded the metal quarter aggressively and re-primed with 2k. that cracked, plus a crack in the plastic bumper. sanded the metal bare, sanded about a 1/2" down on the plastic (where crack was) more aggressively, re-epoxy and primer and cracking happens, in this lower area of plastic. Tried again. got these longer cracks after a week (appeared a day later). Don't mind the rough bodywork and overspray dust - this was an experiment with rattle can primer this time around to see if it would still crack.

That crack is a split in the underlying paint that hasn't been completely sanded out. Sometimes the split continues into the plastic itself. If you don't sand down to bare plastic it will keep coming back, no matter what you put over it.

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so it continues. i appreciate the responses and advice. if its an existing crack under the paint i dont think it would show up in a new place each time i sand more and recoat, in both the metal and plastic.

it almost seems to be incompatibility with one layer of original paint. i resanded the above area and several inches down below it to bare plastic and blended my sanding in nicely with the surrounding paint. i respray and get the below photos. it immediately does this, first coat of 2k, making it look like i hadnt blended the sanding at all. is there some sort of odd contamination going on? i degrease with T909NB-01 wax and grease remover, letting it evaporate. am i mixing the paint bad? why wouldn't it do this elsewhere? I am using an almost 1 year old urethane reducer , something sold to me for my base called T-323, in the shopline 2k high build primer. but again, why wouldnt i have issues elsewhere. I have noticed in many places spraying this car, primer or epoxy sealer, the paint craters (reverse of bubbles) in many areas. took me awhile to get rid of that issue. Paint shop thought may be silicone contamination from some sort of paint car products from original owner (its a 2016 BMW M4).

third photo is same area as 1 and two after another go around of this time degreasing with carb cleaner, sanding smooth, degreasing with w&g and carb cleaner, and painting. tried the carb cleaner to be more aggressive.

Sand it down. Start with a coat of epoxy primer.
Whatever you are spraying over is not compatible with your 2k primer.

First pic looks to be underlying original paint is not sanded back and feathered properly note the Sanding scratch in plastic in first pic solvents in the primer are lifting edges of original paint not feathered back and not sealed properly before primer. Subsequent coats/layers just cover the lifting and sanding scratch.

All should be removed, all damaged paint should be feathered back with very fine sand paper like 300-400 grit you want it very smooth very fine you should feel zero imperfections with your hand, their should be zero sand scratch evidence before sealing with epoxy and look perfect in just the sealer coat but just the wrong color IF it doesn't its not fixed yet.

All bare plastic should be cleaned with Alcohol first then washed with weak but strong solution of soap and water such as Tide or dawn after its fully dried then maybe alcohol again (MAYBE) then a coat of Epoxy let that fully cure it should appear perfect just wrong color then scuff it with red Scotchbrite then clean again before Priming.

NEVER USE CARBURETOR CLEANER!!!!!!!!!!!!!

At this point I suspect you have sanded into the plastic Heavily (This is BAD!)
I wont say its not repairable BUT as a Newb you got yourself into a real mess.
Plastics bumpers are a Pain even for experienced painters, Due to the fact that Not all Plastic bumpers are the same meaning depending on year and make and model they are all made from different types of Rubber and plastic so repair methods & Products used can vary wildly!
What I describe is a VERY Generic method of cleaning and repairing for paint. I strongly suggest finding a copy of the BMW body repair manual for your year and model for direction and repair methods/products to use.

I use Polyvance methods of Identification & repair techniques and then their products!
for all my plastic needs. They have very special products not only for plastic/rubber repair but for painting as well I.E special Fillers, special Primers, special adhesion promoters, special solvents, special cleaners etc. Plastic bumper repair SUCKS! and can be very expensive its very frustrating even for experienced painters.

I own an M car myself and at this point if I were you I would look for a good used original UNDAMAGED meaning no scratches or gouges in it NONE! then just scuff it and paint it your color.
Their is a reason you see so many Minor damaged ones on ebay. OR you can Bite the bullet and get a brand new one altogether BUT stay away from Aftermarket el'cheapo knockoffs their fit and finish is hit or miss at best & the cost savings not worth the effort and frustration to deal with.

Any M car is worth fixing right unless it has a salvage title?

Just my 2 cents

Oh Wow! The better photos tell a completely different story. Yes, that's edge mapping, not a crack as I first thought.

Follow Dennis' instructions to sand smooth. Put your first coats of primer on lightly, allowing plenty of time for each coat to flash off so the thinners doesn't soften the feathered edge of the old paint.

thank you for your help. I am a "newb" but this is my 5th car to paint, so not a virgin.

So i tried again, but starting with epoxy first, as I did the very first time for the whole car, and then the second time in this area because of this issue, not the times described above where I just used primer. i cleaned the above with alcohol, sanded 180 grit on a da to get rid of the majority of the primer and bubbling, then blocked with 220 and 320, then 400 on the da. Cleaned with alcohol, w&g, alcohol. sprayed a light coat of epoxy and waited 15 minutes. It looked ok I think. Put on a second full coverage coat of epoxy and within minutes have the below (after the sanded condition photo). You can see the irritation happens more or less in one of the intermediate layers of white.

That's edge Mapping for sure! I hate that when it happens always a pain.
Something their does not like the epoxy, Probably some sort of 1k product in the layers.

One way is to fully strip it BUT I strongly suggest against it.

Two:
Sand it back down when it cures in a day or two, then Do a few LIGHT coats of epoxy.
Allow a light coat to fully dry before applying another light coat and let that fully dry.
Like an hour or longer between light coats, longer being preferred. It could take 4-5 coats or more before you get it covered That's how light you wanna spray it.

Epoxy takes a Long time to fully dry even longer to cure.

Dennis,
Maybe a silly question, but if I'm painting several coats over several hours, i should empty and clean my gun between every coat, right? The epoxy has an 8 hour pot life but i would think the parts in the gun internals cant sit that long without drying/clogging.

Thanks for the help! Sanded last night. Spraying this evening into the night.