I didn't even use a filler other than many coats of clear dope on two I finished and they both came out great.

When using polyspan for your covering and after the final doping, is a primer recommended before painting?

Mike

Mike, I am no expert at this, as I have only used polyspan once on my Cardinal. So, I had the same questions as you. I used nitrate to attach and it went on great, but I was caught a bit by the difficultly of pulling around corners or curves, like the wing tip. But after a while I got it OK. It states in the instructions that polyspan doesn't go around corners very well, they are correct. I used a little heat from hair dryer to shrink, be careful this stuff melts really quick and is kind of difficult to patch.
I only used butyrate over bays to seal. And only used my main color white, no primer. I think it turned out pretty good. Hope this helps.
Jim

I have recently gone back to a dope finish and have been using poly span. I use nitrate dope and don't worry about it not being fuel proof as the color coats are that. I have been shooting primer on mine and then sanding with 320 wet. A tedious process and you have to be very careful not to sand through. From now on when I build a wing alot of prep work will go into EVERY part that the covering touches. No sharp edges. I have not had any problem covering the wing in one piece, tips included. It is a bit more work but worth it. When I have sanded through and have had to patch I use silkspan. I tear the patch, never cut with scissors. Makes it easy to sand and almost disappears.