rexpontius

I have almost completed disassembling my 86.5 for paint. The front bumpers have some spider cracks in the paint, and I Recall some fellow rennlisters removed all old paint by wallnut blasting the bumpers.

The other alternative would be to sand them down to the bare plastic.

Any recommendations?

Cheers
Michel

GlenL

It's not necessary to get to the bare plastic but you do need to get the top-coat paint off. There are two layers of primer. Getting once the top coat is off it'll look like an agate rock (if that translates) with patches of dirrent colors.

I tried bead blasting but it didn't do anything. They bounced off the soft plastic. Take it to a few places and have them try different media. I ended up sanding it. Worked great in the end. Something like 8 hours per bumper.

88porsche928

For plastics on quads and dropped bikes I use to sand plastics down with a rough grit and then not so rough. But I put several coats of a chromate containing epoxy primer. It would stick like white on rice on the plastics and dry fast. Then after it dried I would scuff sand to make it smoother and paint the color on it, let it dry and sand it dull. then paint the final coats on it and then clear coat it. I wish I had a paint booth in NY i could start painting things again (moved from san diego).

Mrmerlin

if your repaint ing your bumpers and there are spider cracks the best way to remove the old finish is to walnut media blast them,
this will do the least amount of damage to the substrate.
if you try to sand down the finish of the top coat then repaint you will after a few weeks time see the old spider cracks coming up through the paint, this has been seen more than a few times.

If dont follow these instructions then you will be repainting the covers, also make sure to use a flex agent in the all of the coatings used on the bumpers

Ad0911

Will chemical peinstrippers do the job?

Ad0911

Paintstrippers of course. F*****g ipad...

Mrmerlin

dont use any paint strippers it will eat into the plastic and make it soft

Brett Jenkins

Be careful if you power sand them down, it could heat up the plastic and distort it.

tmpusfugit

Dean Fuller has the most recent hands on experience with paint removal from the bumpers that I have seen. He first tried walnut shells, but it did not work very well. He then moved to a more abrasive mix, I think he ended up using aluminum oxide, and it worked well, removed all the paint, and he got a really nice finish on his son's bumpers....you might want to do a search and verify what I think I remember, I do remember that walnut shells did not work....

Mrmerlin

I think the blast media may also have something to do with how many coats there are and the hardness of the paint, i used walnuts and it worked fine others have had to use more aggressive media

Podguy

I have used walnut shells on all the bumpers and aluminum parts I have refinished. It gives a nice surface for finishing. I had suggested soda blasting but have not tried it.

For primer I use SEM plastic primer. It flexible and easy to use. It fills well and sands easily. For cracks I use universal fiber plastic welding rods from Urethane Supply Company. the rods are about a $1 a piece and can be used with a solder iron. They also have two part plastic fillers.

There is no reason to use flex coat in the color coat. It is thin. But you will need flex agent for the clear coat.

Power sanding and chemical strippers are a no no. The chemical strippers will melt the plastic and the tools will leave scars. Sanding is counter intuitive. Most things when you sand them they get lower. The plastic is molded under pressure and has a surface tension. As you sand the material becomes thinner and bulges. The more you sand the worse it gets.

The front bumpers on the older - pre S4 cars - can be reinforced from behind to remove the ripples that generally occur with age.

Cracks can be fixed with both wire and the plastic welding rods. Drill holes along each side of the crack and use some wire to pull the together. Fill with some plastic filler.

Good luck