Yes I have done a lot of block sanding of the first and second thin primer coats that were guide coated. I have used, by hand, the long board with 180 grit as per Rubber Seal instructions with no burn through. I understand your explanation of use of heavy build as giving me more primer to sand off to get the surface smooth without burn through.
Sounds to me like I can continue priming with thin, spray able coats to get same result as one high fill coat.
Yes I will be using Medallion sealer activator 777-8 . Glad you in California know about Rubber Seal made right here in Dayton, Ohio. Must be good stuff. Thanks, Frank