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Old 05-11-2007, 02:21 PM
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Trouble with Pre-Primed Plastics

Hey guys, im new to the boards, and I need a little painting advice. Last week a friend wanted me to paint a front bumper for him from his 2002 audi a4. He gave me the bumper that was supplied to him by the dealer. I wasn't sure if it was primed already or not, but i began to scuff the surface with a scotchbrite pad to prepare to spray my own primer on. Turns out it was already primed. Usually I clean parts with wax/grease remover prior to painting. I guess the scuffing of the surface allowed the wax/grease remover to penetrate somehow, and it began to lift the factory-applied black primer. So I was stuck cleaning the factory primer off the entire bumper by wetting areas with solvent and cleaning the gooey mess off. How can I properly prep a part like this in the future without making such a mess and wasting so much time? Should I use dish soap to clean instead of solvent or wax/grease remover? Should I be sanding the surface with like 400 grit before priming? Or will scuffing do the trick? It was smooth for the most part since it was new.

Thanks in advance,
Brett

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Old 05-11-2007, 05:16 PM
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this is just a guess but it was probably primed with a waterborne primer. plastic bumpers should be wiped down with waterborne wax and grease remover. basically a mix of rubbing alcohol and some other solvents depending on manufacturer. just scotchbrite it next time then base/clear. the guys that work with this stuff every day may have other suggestions for you but thats what i found when i recently did an 06 vette bumper.
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Old 05-11-2007, 05:44 PM
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Maybe this will help.

Primed Plastic
http://www.sherwin-automotive.com/me...d_plastic.html

Unprimed Plastic
http://www.sherwin-automotive.com/me...d_plastic.html
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Old 05-11-2007, 08:29 PM
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Quote:
Originally Posted by mrcleanr6
this is just a guess but it was probably primed with a waterborne primer. plastic bumpers should be wiped down with waterborne wax and grease remover. basically a mix of rubbing alcohol and some other solvents depending on manufacturer. just scotchbrite it next time then base/clear. the guys that work with this stuff every day may have other suggestions for you but thats what i found when i recently did an 06 vette bumper.
that definately makes sense. i wasnt aware of these types of degreaser for paint use. i guess simple green would classify as a waterborne wax and grease remover? i checked out those links bloverby posted and they are informative as well, but they seem to reccommend cleaning off all the factory primer if it is soluble. that is just way too much work though, so ill try using waterborne stuff next time and scuffing.

i am wondering how much work is really required for a cheaper refinish job on previously painted parts. say i need to repaint a fender (previously painted and cleared but very weathered). should i sand up to 400 or 600 grit before putting on primer, or will a nice scuff job do the trick?
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Old 05-12-2007, 08:44 AM
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Don't use simple green.
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