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I am working on a 1986 Monte Carlo for my friend, There is a crack coming out of the lower marker light that sticks out farther than the rest of the plastic. I filled the crack with plastic repair but I don't know if I should try to build up around the area or sand it down, I know that sanding down will make it thinner so I really don't know what to do at this point. If you guys can point me in the right direction I would appreciate it.
On something like that I usually repair the crack from the inside with some reinforcing material, then sand the outside down level, which should leave it about the same thickness.
Cracks have to be veed out and also stop drilled, or the repair won't last very long. Also prepping and building up the back is a good way to build strength, using a bit of fiberglass mesh drywall tape buried in the repair material adds strength to the repair on the backside.
Simply find the end of the crack and drill a small hole through it. That will help ensure that the crack doesn't "run" at some point in the future. If the repair material you have is a two-part epoxy or urethane, you can clean and rough up the backside and use that. I don't think your post says what you are using. I would not use the flexible polyester products to fix a crack.
You'll have to be more specific about what kind of plastic bumper repair material you have. There are dozens if not hundreds of different products!
I've used the SEM 39767 material, and it is good stuff. When doing the top side, make sure the cracked area is made low enough that when you sand it , there is repair material extending 1/2 - 3/4 inch past the crack. This will make the repair stronger, and there will be less chance of the repair showing later on. Crash's advice on drilling the end of the crack and using repair cloth on the backside is also necessary for a long term repair.