Hi Dave
In answer to your questions:
1. "Flatting back" means sanding them down with a wet and dry sandpaper. I started off initially with a 600 grade, and ended up using either a 1200 or 1600 grade to finish.
2. A degreaser is a chemical to remove any trace of oil or other contaminants, I used
Meguiars Super Degreaser but any will do.
3. I only oven baked the initial coat of plastic primer, but following that I had all sorts of paint compatability issues, this was probably a paint reaction, but I couldn't be sure so didn't bother with the oven again!
4. Any bodyshop wil do it I'm sure, but it's quite labour intensive, so if you were going to do that it would be as well to just buy a pair, unless you can get mates rates! Why not give it a go? Pick up a set of any colour mirror caps from ebay, mine were £10 or £15 I forget which, and just try! If you screw it up just sand off and start again. I found it a good little project personally.
Any questions let me know mate :thumbsup: