Here's a really easy project to illustrate the process. These are rear pillar trims from a 5th generation El Camino. The project is on the bottom, on top is what it's supposed to look like.
This is the area where the front and rear pillar trims overlap -- there's a thinned-down area in the plastic where the front slides into the rear. It's so thin that it is prone to disintegrating.
Note how bad the face is -- extreme surface cracking and powdering and loss of material.
The first step was spraying the face with SEM Color Coat to stabilize it. I sprayed the back too, but this was a mistake -- the back was very stable and very smooth -- what I should have done was roughen it up to give it a tooth for the resin to adhere to. The repair still worked fine however.
Note that I am changing the color of these trim pieces from black to Camel Tan.
Next, I made a very simple "mold" for the resin using masking tape. The glue on the masking tape acts as an excellent mold release, meaning that the tape peels right off after the resin cures. I did my best with the tape to make the profile that I wanted, but it doesn't have to be perfect. You just need to create a scaffold to build on with subsequent resin applications.
I poured a layer of resin on the tape. Then I laid down a single thickness of glass cloth, and topped it off with more resin.
I later decided that I should have over-lapped into the good area of the original trim more, which I did during a later resin application on a different piece.
After removing the masking tape mold. The piece on the left is finished except for sanding and shaping. The piece on the right had an air bubble and a void that I filled with left-over resin while working on a different piece, and then it was also ready for sanding and shaping.
The resin that seeped under the masking tape is a good thing, further stabilizing the old plastic and integrating the old with the new.
Again, note how badly deteriorated the original pieces are.
The finished repairs after spray-painting with SEM color coat.
The SEM is remarkable stuff, and is much easier to use than paint. I really, really soaked these pieces in order to fill in the cracks, but it still looks really rough. I considered using Bondo glazing compound, or some other compound or surface filler to skim coat the entire piece, but decided that would be too much time and material and trouble, especially for a piece that you can still buy new or on eBay.
Anyways, I'm happy with the repair, and it sure looks a lot better than it started out.
If you had an absolutely irreplaceable plastic piece that needed repairing, this would work. With enough time and patience, and using glazing and finish-filler compounds, any plastic piece could be made to look new.
I'll show a couple more repairs that I made, and will talk about the materials that I used and how I used them in up-coming posts in this thread.