Cracking

danp76

danp76

Oldtimer
I installed a new outer wheel well on a 56 chevy convertible I'm working on. I painted it in spi black epoxy and them I seam sealed it a few days later with urethane seam sealer. I then proceeded to spray black under coating over the seam sealer and where the outer and inner wheel well meet to blend in the old undercoat with newly installed panel. I was looking at it today, and the undercoating is all split and cracked looking...what gives?? Is this a result of the seam sealer gasing off or the epoxy gasing off? I waited a few days before seam sealing and undercoating.... I don't want the same thing to happen to the opposite side, as I'm almost done with the welding..thanks guys!
 
Cheap undercoating-shrinkage? What seamsealer did you use? What undercoating? Given the few days cure time you shouldn't have any problems that would be related to the epoxy or seam sealer if it's a true urethane sealer. You'll see this kind of thing happen with the old style solvent based sealers when they soak up solvent/expand/then shrink.
 
I've seen cheap rattle can undercoats crack like this. I've had good luck with rustfre brand undecoating that comes in a gallon. Seems to last a long time and looks like a nice job when completed compared to rattle cans.
 
agreed with bob. i use the 3m ultrapro 1 part urethane seam sealer and fusor 800ez 1 part urethane seam sealer and have never had anything crack over it. have 3 cars with spis bedliner under it, over either one of those seamsealers.

the quality urethanes can take paint or any other coating within 30 minutes...i've done it in less with no problems but only in a pinch.


also used the 3m shutz undercoating over these sealers many times with no problems.
 
Is the cracking only over the seamsealer or averywhere? Sounds like you had some big time shrinkage with the undercoating. If you're applying a lot of product to try to duplicate that rough looking oem stuff that would be the problem-too much material and when it purged the solvents it shrunk so much it split. You could use the 3M Bodyschutz in the orangish colored quart can and spray it wit a schutz gun-this matches that oem heavy stuff pretty well.
 
probably what happen as you describe Bob...is there anything in an aerosol I can use?...I want to "blend in" to the old oem thick undercoat...can I go over the splits and cracks with other stuff?
 
The seam sealer sucked the solvents out of the undercoat and expanded, cracking the fast drying undercoat.

For next time, if you put a thin coat of epoxy over the seam sealer, then come back with the rattlecan undercoat 24-48 hours later, there will be no problem.

I have shot over those kinds of cracks, since the undercoat is 1K, the subsequent application will melt into the cracking and it shouldn't be prone to showing later.

But, depending on how nice of a job you are doing, you may wish to wipe the cracked stuff off with solvent rags to make 100% sure it won't show.

I use liter cans of Wurth undercoat and a schutz gun on nice jobs.
 
I've tried the waterbased Wurth stuff and didn't care for it. Hope 3M doesn't quit making the old solvent stuff.
 
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