Andrew,
what sort of holes do you mean?
Big holes (like a cutout for a removed diesel ventilator housing): Cut a piece of styrene sheet to fill the hole, then glue it in place with thin strips of styrene below the gaps to give enough glueing surface for holding the pieces together. Most probably there will be more or less visible gaps between the car shell and the inserted piece. This brings us to...
Small holes (gaps, holes for removed detail parts like diesel horns...). I'll second Gary here: Use a good brand of plastic putty - like Squadron's Green Putty. A very good source for that stuff is a hobby shop which specializes in plastic model kits.
As Gary said, putty shrinks somewhat. Therefore it is important to put not too much of it into the gap/hole. Let dry, then apply some more etc., until the gap is slightly overfilled. When dry, sand down the slight ridge. Needs perhaps a little more time, but finally turns out just perfect.
However, I for myself wouldn't go for the method with dissolved styrene. I tried it a few years back (when I was modelling racing cars), and boy, what a mess I got! Might be that I made something wrong, but I ended up with spiderweb-like styrene threads all over the place, sticking to everything - and almost ruining my model: Instead of getting hard, the self-made-goo softened up the parts of the kit, so the modified motor hood began to droop...
Perhaps somebody of you Gaugers had better luck with that stuff. If so, please let us know how to do it properly!
Ron
