Anything goes in the world of fiberglass and plastic
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This is a post copied from another topic, but the question comes up often enough that it merits being a 'sticky'. Some notes regarding how I've handled bumper repairs...
The 3M collision website is a great place to go look at and research the products for lots of different types of repairs. Look at this section on plastic repair. I'm a big fan of 3M products and have used them many times on these repairs.
So, this is the way I do it. Others may do this differently...there are always a few good ways to get to the same end result. I've had bumpers that were basket cases turn out great (and probably spent too much time on them instead of hunting down a good replacement from a salvage yard! )
The bumper should come off the car for this.
1. Clean the entire bumper really well with a mild soap/water solution.
2. Clean the entire bumper with a wax/grease remover. In the following steps, after each sanding step you should blow the part off and clean with w&g remover again.
3. Sand the damaged areas (where you will be applying a repair epoxy) with 80 grit. Feather out any areas where the paint is damaged but the bumper itself is not damaged, with 180 grit. Also, around any of your 80 grit damaged/sanding areas, feather out into the paint around that with 180 grit as well.
If you have any crack-throughs in the bumper it needs to be plastic-welded, or at a minimum bridged with a repair patch like this one, from the back side of the bumper.
4. Apply 3M flexible parts repair epoxy over the damaged (80 grit) areas, but first you need to spray a wet coat of poly adhesion promoter
5. Block sand the epoxy repairs down to 180 grit, then spray another coat of adhesion promoter and apply a skim coat of epoxy putty again over the repaired areas, and any areas where you had previously sanded with p180 grit paper.
NOTE: Regardless of a/b choices below, any repaired areas will need either a sealer or a primer, before base color. Safest bet is to just apply sealer over the entire bumper.
6a. If you are NOT going to shoot a sealer coat over the entire bumper before color (e.g. if you are shooting a dark color or a color that achieves excellent hiding with no concern about color differences on your substrate): Block sand all the repaired surfaces of the bumper through p220 grit, then p320, p400 then finish with p600 if you are shooting metallic paint (stop at p400 for solid colors). Feather into non-repaired areas. Any areas of the bumper you didn't have to touch (i.e. still has clearcoat), sand with p600 or use a 3m Scotchbrite dark grey pad.
6b. If you are going to shoot a sealer coat over the entire bumper before color (e.g. if you are shooting a light or translucent color that will appear blotchy if the substrate is different colors): Block sand all the repaired surfaces of the bumper through p220 grit, then p320. Any areas of the bumper you didn't have to touch (i.e. still has clearcoat), sand with p400 or use a 3m Scotchbrite maroon pad.
7. Shoot a coat of adhesion promoter over all repaired areas again, let it dry then lightly scuff the entire bumper one more time with a Scotchbrite pad. Blow the dust off the bumper and tack cloth it clean. Don't use w&g remover at this step.
8a. If NOT shooting a sealer, next shoot your base color over the entire bumper, a couple of coats or until you have complete hiding/coverage.
8b. If shooting a sealer, apply one or two thin coats...we don't want a lot of millage buildup on a flexible bumper. Also check your tech sheet for the sealer and make sure it's ok for flexible parts. After sealer, proceed with base color as noted in step 8a, above.
9. After specified flash/dry time, tack-cloth the bumper off again then clearcoat the entire bumper...two coats should do it.
Now if you really want to try to just touch up the repairs, and don't care that it will not look "new" -- you can try to feather out the damaged areas with 320 grit paper, spray adhesion promoter then spray some of that cheap aerosol touch-up paint from the auto store...and hope for the best.
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