Frustrated with inability to do body work!

George, first thanks for getting this thread back on track. When you say long board I have a set of duro blocks that are up to 12" long. Also bought a single one that is 24" long. Im assuming thats what you meant. Im using the 12" the most. Once I put the filler on and wait for it to harden then I hit it with the 80 grit. Thats the most course i have. Then Ive been going to 180 grit. I think Im getting close to being ready for primer so I can see where Im at. Heres a question I have, once I spray a coat of primer and then use guide coat can I put more filler on top of primer or do I need to sand back down to bare metal?
Thanks Rod