I actually used the dremel on a part of the car that would be concealed. I'm currently building Aoshima's S-15 Silvia Spec.R (#46). The kit doesnt have an engine but the hood comes off so I tried the dremel on a small area under the hood. With my amateur modeling skills I wouldn't dear run the dremel on any exposed parts of my models.
Some people have written that they melted the plastic because of the high speed their dremel was running on.
The picture of the mini cooper Klutz has at the bottom of his messages has a clean paint job. Something like that would be my goal.