I happen to use micro balloons and finishing resin - will try RC's cornstarch suggestion.

Like others, I create a very dry paste. I use it to fill around control rod tubes and build up wing saddles and shape around the leading edge of the wing. Use wax paper to isolate the putty from the wing. I don't use all of the paste but reserve a dab leaving it on the wax paper. I take the dab back to the real world and keep tab of how it hardens. When it gets beyond "refrigerated cookie dough" consistency I go back down to release the wing from the saddle and remove any wax paper. At this stage this stuff is firm enough to hold its shape but soft enough to be cut easily with a razor blade or x-acto knife - don't overdo the trimming 'cause it sands pretty easily when dry. After trimming I leave it overnight to harden before sanding.

Cheers,