Phil,
I waited 3 days - the proseal wasn't fully cured (I even brought the parts into the warmer basement than the garage shop). It was enough to keep it straight when I backriveted - just follow Van's rivet sequencing recommendations and hold the elevator (or rudder) flat on your work surface. I actually shimmed the elevator on wood planks the same thickness as the steel back-riveting plate, and slid the plate side to side under the trailing edge to rivet. Some folks have put on weights to the elevator to hold it flat (lead shot bags, sand, boards), but I held it as I riveted and frequently checked the TE. A tip on removing the angle that will likely be stuck to the TE - I rolled it off from front to back to unstick it so it wouldn't debond the trailing edge.