I think the problem you guys have with the milliput and filler, is that you might being putting it on to thick.
Milliput is best left for 12 hours or so to harden..then it should be dead easy to sand with a 120 then a 240 wet an dry..Then gradually go down a grade to get a smooth finish..
You really should not need a dremel to shape filler on a 1/18TH scale car