Hi Gordon,
It's funny, you guys always ask the obvious questions that I forget to discuss. I have the same problem when I'm teaching guitar to beginners, sometimes you just assume knowledge that isn't already there.

onto your questions:

1. Probably the biggest oops I made is forgetting to mention the short window of opportunity you have to polish clear coat. Most clears require 24 - 36 hours to cure hard enough to polish. However, your window of opportunity is between 36 and 48 hours. If you wait more than 48 hours your clear will be to hard to polish all the scratches out of easily. In that case you will have to run a higher RPM which will increase the temperature, the clear however will not burn as easily at that point. The best shine comes from using the correct products, and using those products correctly! If you get fresh clear to hot, it can streak, basically it will ball up the clear and smear it. If you get it to hot, when it cools it will shrink and you'll see scratches again. The biggest problem on our models, is if you get it to hot, when it cools off, you will see the fiberglass weave, or internal structure. This also happens if you are applying more pressure than just the weight of the machine. The polishing removes a very tiny amount of clear from the surface. When the fiberglass gets hot it becomes softer, with pressure when you go over a former or other piece of structure, the fiberglass cannot move away from there, so you remove more clear from that area than the surrounding area's, in the end, you see the structure. Keep the surface cool, and use no more pressure than the weight of the machine, and assuming it is good fiberglass work you won't have those problems.

2. When I buff the surface I work an area that is generally about 2 feet square (pretty easy to do on a model, other than the fuselage there aren't a lot of area's bigger than 2 feet square). I am continually moving the buffer over that area, this keeps the heat out as well as keeps me from spending to much time in one spot and ending up with inconsistent polishing (some area's buffed more than others resulting in uneven shine). When I move to the next area, I overlap the first area a little bit and continue polishing. I move at a medium rate of speed I suppose over the surface, it probably takes me 8 to 10 seconds to go over a 2 foot length, then when I come back, I move the buffer so the polishing pad is overlapping the first pass by approximately half. When I buff the wing panels I'll try to maybe do a quick video.

3. As I mentioned earlier, I do not thin my clear anymore than the manufactures recommended amount. By spraying 2 wet coats as I described, you are not putting that much clear on the surface, but it is still more than enough to sand out tape lines and polish without going through (as long as you are careful). When you over reduce your clear, you are breaking down it's polymer structure, it will not cure as hard, this makes it easier to polish through all by itself along with a reduced film build.
As for tips or tricks on how to tell when you're about to go through, there really aren't any outside of using a paint meter to measure the thickness. But one that will work on fiberglass or plastic is very expensive (costing a few thousand dollars) and on our models I'm not sure how well it would really work for that application anyway. You would need to calibrate it before spraying the clear to know how much clear is on there and then you would have to make sure you measure in the same area all the time, so really, you wouldn't know anyway if you were thinner in some spots than others. Your biggest risk for burn through is on edges, or tighter radius curves. Flat surfaces are pretty safe, you'd almost have to try to burn through on those.
Jeremy