How thin can it get?

I'm not sure that thinner is going to give you the effect you want. It's going to react to the plastic much the same as liquid glue does and you have no control over how deep it goes. And unlike photo etching metal, the plastic is not removed, just soft and laying there. If you then try to remove it you've got a real mess. The only times that I've seen or used thinner or liquid glue in this manner is when a "cast" surface is desired.

I would submit that the best ways to thin plastic are:

A. What the FAA refers to as "mechanical means", ie sanding, scrapping or fileing which gives you far better control over the final thickness.

B. Removal of the part and replacement with a thinner piece or a more accurate scratch built part. (any modeler who has lost a part to the 6th dimension or to the "Shag Rug Monster" learns how to do it)

I concur with Qmeister.

Do not use paint thinner to create thinner plastic parts.
You'll just get a plastic mess!

Use a grinding bit in a Dremel tool to thin plastic, but use a slow speed, or you will melt the plastic.
Lee

The only thinner I could see working would be laquer thinner, which is bad for your skin, bad to breathe and yes, would make a big mess. I would stay with the above mentioned methods.

When I read people saying that they thinned their fenders and stuff like that I took it litteraly and thought they used thinner to make their fenders thin and realistic. I guess it was a simple question that I made complicated, thanks everyone.