I teach plastic repair and have done repair in body shops for many years. Some things you just cannot repair and have it be strong enough. as in small areas. You need surface area built up on the back for strength.
Cleanliness is extremely important. There are many types of chemicals sprayed on our cars, almost all of them will hinder adhesion. Clean, clean, clean with soap and water then a quality automotive grade wax and grease remover Do not use lacquer thinner, toulene, glass cleaner or any other products. The soap and water removes contaminates that are water based, the wax and grease remover removes oil based contaminates.
Then you need to sand the back of the repair area with 80 grit. Complete sanding with no shiny plastic showing in an area much larger than the damage. Strength is gained by thickness of repair material and surface area covered. I use clean 80 grit glass beads in a blast machine. You need to get all of the repair area sanded. You need 80 grit to provide sufficient "tooth' for the adhesive to mechanically bond to. This also actually increase the surface area, up and down the scratch valleys is more surface area.
After sanding clean again with wax and grease remover or automotive alcohol cleaner. Your serious contamination should have been removed long ago, the alcohol will be sufficient and it will kill some of the static from rubbing the plastic while sanding. To clean properly with liquid cleaners, you flood the area and wipe it with multiple clean dry rags. I use rags in a box for non painting cleaning. The goal is to get the contaminates to float up into the liquid and wipe it away, not smear it around.
JB wed is almost worthless.
I use 3m 8115, 4240 or 4248 depending on how long of work time I need. Despite what was posted, in addition to the mechanical adhesion from sanding you want chemical adhesion from the repair material chemically bonding to the plastic. This is why I use as slow of curing product as possible. The longer it takes to cure the more it will sit on the surface and "wet in", increasing chemical adhesion. Clamp, fixture etc the part so you do not have to hold it waiting for curing. Sometimes I will use superglue only as a means to hold it while applying and waiting for a 3m product to cure. I also have a tool that melts in a metal staple for a temporary hold. You could try the same with a T50 type staple upside down and clip off the legs before applying adhesive. If you melt in something to hold it or use superglue, you must sand and reclean those areas. Adhesive will not stick to the melted, shiny plastic from melting in a staple. You can add some reinforcement if you want, fiberglass mat (NOT cloth) or even drywall mesh. adhesive does not stick well to a steel screen unless you sand it 100%, possibly glass beading would work but the drywall mesh type is better. I use Fusor #700 plastic repair mesh if needed.
You gain strength by proper surface prep and by making the repair larger then the damage on the backside. Make the backside repair bigger and thicker in an attempt to make it as strong as the original piece.
Often if it is just a crack the frontside damage will be almost invisible it the crack was aligned properly. If not front side repair will need to be done which is not the same as the rear and probably a topic for another post.
We will be working on an 88 RCSB this spring, if we have any plastic repair on the interior pieces I will try to do a video or pics. I am trying to learn how to do videos. I am sure the interior trim pieces will come out with no breakage!
