Today's lightweight plastic fillers are easy to use, flexible, and have excellent adhesion. The key to getting them smooth is to keep the material uncontaminated. Always mix on a perfectly clean surface, and use a clean plastic spreader with a good smooth edge. Mixing on top of a previously mixed batch, reusing the same stirring stick, or having dry residue on the spreader will invariably drop lumps of dried filler into the mix, which will plow in the filler while applying. Never use the same stick to scoop unmixed filler and stir in the hardener.