i use only zap finishing resin.first coat thinned with denatured alc{10%}.spread with a playing card.take all you can off. like putting on a decal. let it dry. longer is better. on the second coat a mix,, 30% Q-CELLS. ,,NOT MICRO BALLONS..,, mix just enought of the cells to turn the resin white. it should flow like ketchup.spead it with the playing card again.this will flow out smooth.sands like balsa. you can go to 400 grit if you want.not much primer neede. just to help the paint to stick.ballons leave holes the need filling,cells dont.also much less weight with ,, q-cells.- if you want to save a little more weight,use .. WARP CLOTH.. has moe fibers in one direction.perfect for wings..