Thinning makes it easier to apply smoothly requiring less sanding. Two coats to fill the weave with sandable primmer on top. Second coat add microballoons to help fill the weave. Easy to sand, and Weighs less too. Here is a link:

http://www.rc-tech.net/

Click the fiberglassing link.

Fiberglassed this about 7 years ago. Holding up well:

http://www.rc-tech.net/gear/firsta.jpg

Gary