How do i make smooth cuts on body

[mistercrash]

If you have a steady hand with a hobby knife, put a new blade in, and score along the line of the wheel well of the body, or the lines you draw with a sharpie. You want to score the Lexan deep enough so that it will cleanly snap off when you bend it at the score. That's how I've been doing it for years. In my on road days, I used an OLFA circle cutter as the wheel wells for on road cars are mostly perfectly round like the wheel itself. I still use my OLFA circle cutter to make vent holes.

That is just another way of doing it.