Hi Bob
Don't try too use any filler over polyspan untill you get at least 3 or 4 coats of clear dope, I recomend spraying this only to keep it on top and the same thickenss, if you sand before that it will fuzz and you will have lots of greif, After your surface,all over is sealed and sanded flat, I use it this way,use the ZS filler mixed up in a qt jar 1/4 thinner 1/4 full unthinned dope, mix in 4 heaping tablespoons ZS, mix this as well as you can, stir shake, then fill the qt jar all the way up with the balanced of unthinned clear, put about 4 or 5 drops of black paint in it so you get a light grey.
pour off 4 ounces or so in your spray gun...I use a dedicated cheap qt gun that I only use for filler, thin it 100%..ie 5 ounces ZS dope 5 oz thinner, spray it over the entire airframe. let it dry and look for bad spots or grain.. brush full strenght ZS filler only over the bad spots, let it dry then sand off the entire airframe, It sand better than anything I have ever used, I use 400 grit gold paper 3M. It takes me 30 min. to sand a complete SV size plane.
Block sand everywhere as much as possible, Do not sand into the paper or polyspan deep enough to cause fuzz.
At this point I spray the entire airframe again. if there are still grain and bad spots brush full strenght filler over only the bad spots and repeat the sanding and spray another coat. You will have the lightest filled surface of anything I have ever used. and I have tried everything!
If you have really bad spots to fill you can use a thick filler only on bad spots and put ZS sealer over that.
The ZS filler/sealer is more like a thick sanding sealer rather than a filler. Also on the last sprayed coat of the ZS filler you can sand lightly to just know off any bumps and use it as a grey base to put the color over.
Need more info let me know.
Regards
Randy