Anyways, talked to the "Customer Care" rep at one of the body shops I took
the car to for estimates, and he was pretty sure that they wouldn't have to
strip the whole bumper before repainting.
A few people have suggested that it needs to be completely stripped, and the manual says the same thing, but I figure there are probably some folks on the list who haven't bothered - if anyone has sprayed over the existing paint, I'd like to get your opinions. Did it hold up? Any long term issues?
Well, I sanded down the bumpers when I painted them. Not exactly "stripped"
but down through the topcoat and into the primers and to
the plastic. Looked like an agate. I did this because the PO had a crappy re-paint done. When I got it the car had an unsightly "spider" crack (12" around) on the front fascia from a minor hit. The cover itself had rebounded.
So you don't "need" to have it stripped. It just works out a lot better that way.
Same here, sanded bumper covers down to that factory amberesque primer that
they used everywhere. It was really well bonded and not flaking so I saw no
reason to have to re-establish that well researched bond. I also used a bond
promoter which smelled and looked like kerosene but was marketed as a solvent
application for urethane systems. Supposed to soften the upper layer to promote
bonding of new materials with old.
Worked well. Also used flex in the primer and in the clear as prescribed by Dupont and it seems to have held up well for quite a few years now.
That is except for areas that were subject to operator failure during the respray ;(
Spider cracks are typical of a repainted bumper. So s peeling clear coats.
The clear coat has to bond to the color coat. If too much time elapses between the two coats then there is not a sufficient bond. If the clear coat does not get a flex agent added it will be prone to cracking and even separates from the color coat.
It costs about $80 to strip a bumper. This is pretty cheap. From the sound of the damage it sounds like the bumper shocks are hosed, so the cover has to come off. It seems like false economy to go to all that work of repairing the car and not make sure the job will last.
For the record, I have never seen a repainted bumper not get paint cracks,
peeling paint and abnormal chips.
One issue on sanding. The plastic on the bumpers behaves odd when sanded.
When you grind down metal it gets thinner and can be pushed lower and filled. When the bumper material is sanded it expands from the lack of surface tension. The more it is sanded to smooth the imperfections the more the surface expands and spoils the look and repair. If they decide to strip the bumper by sanding then they have to be very careful not to sand through the primer. A chemical stripper is a better approach. I have the bumper stripped by being blasted with walnut shells. This gets the surface down to the plastic and then I build it up from there with flexible primers and paint.
The 928 was the most expensive and sophisticated member of the Porsche line up. This is saying something considering Porsche discontinued the car in 1995 and always works at the leading edge of performance and material. While the same materials have found their way into other cars, the 928 still remains a sophisticated car requiring the proper repair by people who know what they are doing. On a scale of one to ten for botched repairs, from what I see of other 928s they seem to rate 8 or 9 in the frequency of bad
Dan the Pod Guy