Yes you can use rondo, but keep in mind two things about traditional rondo though. 1) its kinda heavy. Not a lot heavier, but its got a decent weight to it. 2) its kinda fragle. I know people on the forums who have dropped pieces form a shoulders height or so and have it shatter. What you can do though is use Bondo glass instead of Bondo bodyfiller. Essentially what it is is Bondo mixed with fiber glass, so you get some of the ease of regular rondo, with the the hardness of fiberglass (not as hard, but harder than plain rondo).