So I started with the tear today. I first used a razor blade to ops the tear up enough so that it would easily slip back together. When you get a tear in our bumpers, it more than likely the tear will stretch the surrounding plastic a bit.
Once the tear wasn't rubbing against itself I used a heat gun to reshape the dents close by to it. (A tree had grown against mine leaving a dent)
Next, I took my die grinder and started grinding into the tear itself. I want to open a small gap (~.125") that the panel bonding adhesive could flow from the inside to the outside. I beveled the edges of the tear and sanded the immediate area (about 2") with 40 grit to give the adhesive a solid mechanical bond. Outside of that area, I da'd the surrounding area with 80 grit (to about a 3" diameter).
I cleaned the area with soap and water and a wax and grease remover. I also treated the area with a plastic adhesion promoter prior to using the adhesive. This provides a chemical bite into the plastic as well as the mechanical from sanding.
So, to repair the tear I used a 3M product called Panel Bonding Adhesive (3M # 8115). It's used mainly for attaching new quarter panels, bedsides and the sort. I used this stuff because it's incredibly strong after it is catalyzed. Plus it remains flexible! Good sht!
I also used webbed dry wall tape to reinforce the back side of the repair. You can use fiberglass mesh too, I just really like dry wall tape.
I squeezed out a small amount of the adhesive on the backside of the repair area at first. Enough to hold the mesh tape to the inside surface. After that I put more of the adhesive on a mud spreading board and used a cheap plastic spreader that I had cut down and spread it smoothly over the backside.
Then, on the outside, I put more adhesive directly into the area I had opened up earlier in the tear. I used enough to fill it and spread it relatively smooth over the surface of the surrounding area using the bondo spreader I mentioned earlier. In the pictures you'll notice I spread it over a couple other areas. There where some gouges and gashes there that I filled with the PBA simply because it was easy to do.
Notes about panel bonding adhesive. Wear gloves, oh my god, wear gloves. If you think you've dealt with something that won't come off your skin, you're in for a whole new experience with PBA.
Once it's spread, time to wait. You should wait over night before you try to smooth it, unless, of course, you have a paint booth right next to you that you can heat up to 160 degrees for awhile. Now, where would I find one of those.......?
The PBA can be sanded similar to bondo. I'll use 80 then 180 grit to level it and then use some flexible bondo to finish the repair.
Pictures
Last picture is required refreshment when repairing a bumper.