My front bumper needs a respray as the paint is badly cracked. I'll be taking the car off the road in about a month to do some maintenance and will remove the bumper at this time and get it sprayed. I just want to get an idea of how is it best to prep the bumper for respraying. Ideally I think it would be best if the old paint was completely stripped, how should this be done on plastic?

 

Walnut shell blasting to remove the old paint.

 

Interesting. I could have my work cut out for me finding someone who does that in Edinburgh.

Would normal blasting damage the plastic?

 

What I understand is that if you use different media, it is too abrasive and may warp the plastic. Walnut shell media is the best to get a smooth base without over heating the plastic and warping it.
I know that, walnut media is recommended to get rid of the carbon buildup on BMW heads. So a BMW shop might have more info of a local source.

 

I sanded mine after failing to find a place that could blast it off. Lots of hand work and be careful to not gouge the plastic. Coarse sandpaper can leave marks.

The plastic requires a special adhesion promoter to be applied and then the paint needs a flex additive.

There's more details to be found through search.

 

Lol, i sandblasted mine... Everyone always says "no you'll warp everything with sand". BS... if you're an idiot and hold the nozzle up real close like an idiot yes sand will cut through that polyurethane like butter. When i did it i was surprised how many layers of paint there were, first i cut through the clear coat and base coat, then there's a primer under all that, then a bright green sealer, very thick. Getting it blasted with sand is cheap and gets the job done, i'm doubtful walnut shells would even take the basecoat off, let alone all the other layers of primers/sealers there are underneath that. The bare polyurethane was dark grey when i finally cut through all the layers. Even with the aggressiveness of sand it took a damn long time. Had it painted for $500, looks amazing now! I tried a bag of walnut shells because i wanted something much less hazardous, waste of time. Didn't even touch the cam covers/intake coating.

 

There is a plastic/urethane safe chemical stripper in an aerosol can. Got it at O'Reilly's and it worked great. But as previously mentioned, there are several layers of paint so it took 4 applications for me to get all the paint and primer off. Then used walnut shells to gently remove whatever residue was left.

Cheers!
Carl

 

Ok sounds like there are a mix of opinions. I think I'll drive around some of the body shops ask them how they would do it and get some quotes. Really wish I hadn't leaned on the bumper and damaged it. Looks like a bigger pain in the *** than I first thought.

 

Remember that taking the bumper off gives you access to that part of the headlight washer system, WYIT... The last bolts on the retaining brackets are a hell of a pain though. I spent alot of time trying to undo those last 2 hard to access bolts.

 

I just did mine by sanding. Also, you will need an air powered staple gun to put the rubber trim bead around the edge. Not hard to do, but I don't think I could have done it with a hand powered stapler. New beading is cheap and can be purchased by the meter.

The last two inner nuts near the headlights are a real pain. The ones outboard of the headlights was best done with a ratcheting box-end wrench. For the inboard one I used a 8mm quarter inch drive socket with a long extension. For the life of me I can't imagine why Porsche used those tiny 8mm nuts. Other sports cars I've worked on have been far more serviceable in terms of bumper cover removal.

Some guys have been able to do it without removing the headlights, but it is easy to do and to me was worth the added access. Good luck.

 

I did mine with headlights in, mind you it's an S4 bumper, but pretty much the same mounting points.