1. I never thin it . . .
2. It depends on the type of shafts. PO Cedar and Surewood D. Fir shafts are extremely smooth, so I end up with five coats. The Sitka spruce shafts from Hildebrand aren't as smooth so they take the laquer better and only require four coats. If cresting, I put three coats, crest, then apply two more coats. If you have any problem with the cresting pulling then spray with a spray laquer and allow to dry, then apply final coats.
3. No need to seal, the GL is the sealer.
4. I use 400 grit sand paper and sand my shafts first. PO Cedar it roughen it and to smooth the Sitka spruce.
Holler is you have addition questions.
Michael