Paint just beads up, WTF?!?! - PeachParts Mercedes-Benz Forum

bgkast · #1 02-13-2009, 11:01 PM

I am painting the plastic bumper panel of my W126 black. So far I have filled it, sanded it down and two coats of black on it. After the coats dried I decided I needed to sand some more and in the process sanded through to the old silver paint in a few places. After washing the part again and wiping it down with degreaser I tried to apply another coat of black however the paint seems to "bead up" and leave bare patches in many of the locations where I sanded through to the silver paint.

What gives?

Chad300tdt · #2 02-13-2009, 11:06 PM

Sounds like you need a coat of primer first.

bgkast · #3 02-13-2009, 11:31 PM

Maybe. I did paint all the side panels this way though and they came out fine.

Two nights of sanding down the drain.

Chad300tdt · #4 02-13-2009, 11:34 PM

Did you sand down to the silver on the side panels?

What you're describing sounds to me like paint incompatibility. Self etching primer can get rid of the problem.

TheDon · #5 02-13-2009, 11:34 PM

if its plastic I'd just sand it smooth to the plastic and use krylon fusion

bgkast · #6 02-13-2009, 11:57 PM

Quote:

Originally Posted by Chad300tdt

Did you sand down to the silver on the side panels?

What you're describing sounds to me like paint incompatibility. Self etching primer can get rid of the problem.

Yes, I painted over the silver on the side panels too. This bumper is from an '85, where as the side panels are all my stock '82 pieces...maybe they changed something with the paint formulation.

The thing I don't understand is why my first two coats worked fine.

Does self-etching primer work on plastic?

I can't switch to krylon now Joe, I've already painted half the parts.

Chad300tdt · #7 02-14-2009, 09:05 AM

Quote:

Originally Posted by bgkast

Yes, I painted over the silver on the side panels too. This bumper is from an '85, where as the side panels are all my stock '82 pieces...maybe they changed something with the paint formulation.

The thing I don't understand is why my first two coats worked fine.

Does self-etching primer work on plastic?

I can't switch to krylon now Joe, I've already painted half the parts.

Yes. I've used it successfully before. How flexible the plastic is determines how long lasting it will be. Whenever I paint flexible parts, I use a paint gun so I can mix in the appropriate additives. Except for interior parts anyway ... I use SEM color coat from a can on that stuff.

Mike D · #8 02-14-2009, 09:42 AM

My guess on the lack of adhesion would be a conflict between the "newer" plastic and the degreaser. Re-sand, clean with a 50-50 mix of denatured alcohol water mix, prime and re-paint.

The plastic may have had a treatment which was re-activated by the degreaser.

PanzerSD · #9 02-14-2009, 12:32 PM

Quote:

Originally Posted by Chad300tdt

Did you sand down to the silver on the side panels?

What you're describing sounds to me like paint incompatibility. Self etching primer can get rid of the problem.

Self etching primer is only effective on bare steel.
Plastic, in this case Urethane, requires only a high solids primer and, a primer/surfacer, and then paint.

degreaser on plastic is a BAD idea, it soaks down thru all the layers of paint and into the plastic. there are special degreasers for plastic they are alcohol based.

a Mineral spirit type degreaser should be used before any sanding is done, and the alcohol only after sanding to remove dust and skin oils.

Question: What color is the plastic? is it a sort of light yellow or tan colored? If so, that's Urethane. if it's a grey color or black even, it could be TPO or TPE plastic. which requires an adhesion promoter.

bgkast · #10 02-15-2009, 04:34 AM

Well I primerd the heck out of it today, and it worked fine. I think maybe I just did not give the degreaser enough time to evaporate. It was a mistake to try to skip priming after all the filling that I had to do on the bumper.

The later bumper plastic is yellow, my earlier plasic is black.

I told the guys at the paint supply store I was painting plastic, so I think they gave me a plastic safe degreaser.

TheDon · #11 02-15-2009, 10:26 AM

I usually just use rubbing alcohol to degrease plastics and vinyls. I've never had an issue

bgkast · #12 02-18-2009, 10:31 PM

Well after much priming, sanding, priming, sanding, priming, and painting I got the final coats on it tonight. I was painting it out side and put the last coat on a bit before dark, then left it to dry. I just brought it inside and found it has a few dull patches on it, maybe from dew? ^&(#$!! I hope they will buff out. Other than that it looks great.

PanzerSD · #13 02-18-2009, 11:10 PM

The dull spot might be bleed thru from the several layers of primer "haloing" the areas where two or more coats have been sanded thru. Things like that would typically, in a bodyshop environment, require a sealer before the color coat. Primer-sand-primer-sand, then when beginning the painting process, the sealer is applied then a color coat.

Paint directly on primer doesn't always work. If you were to prime, sand and prime, then after the primer flashes (turns dull) then spray your color coat.

PanzerSD · #14 02-18-2009, 11:14 PM

but if it is dew. it is better it be ontop of the last coat, than between two coats. the paint should still cure underneath. put it somewhere warm tho.

compress ignite · #15 02-19-2009, 12:23 AM

I use the Plastic and leather prep product.Spray on,Wipe off (Lint free cloth)
Last step before applying "coating".Then let "coating" dry overnight(protected)
you will not be able to get the "Coating" off with 91% CnH2n+1OH

Check out their instructions page:

http://semproducts.com/images/Users/1/training-pdfs/Plastic%20Repair%20and%20Refinishing%20Procedures.pdf