I used Smooth Prime ( http://www.aircraftspruce.com/catalog/cspages/smoothprime.php?clickkey=30869 ) 15 years ago when finishing my KR. It seems a bit expensive now but it wasn't cheap back then either. I don't recall how much it took but certainly not the three gallon it suggest in the ad, maybe a gallon and a quart. It is water based and I put it on a bit at a time with a 4 inch foam roll as I finished different areas of the plane. I covered it with cheap auto paint ($40 a gallon in early 2000) and it still looks like the day it was painted. Paint was never buffed out and I still have two spots on the left wing that have been in primer since 2004 where I cut in to the wing to fix a fuel leak. Nobody every notices the primer spots and I've just never bothered to paint them.
The trick to eliminating fill is to get the foam right, get a good glass lay up, then cover with "deck cloth" for minimum fill. If you look "real close" you can see one of the primer spots in the photo. https://dl.dropboxusercontent.com/u/32133949/024_24.JPG Larry Flesner
