2 options, poly or nitro, both sprayed and from rattle cans. You can spray nitro over poly, as I've done it. Need to prep the surface (scuff coat) and get a proper mask if nitro. Lots of stuff on youtube.
You can also spray poly over poly. It pays to practice your spray technique as easy to mess up, again as I've found out myself.
After spraying need to leave to dry ( nitro takes longer), lightly flat and polish up.
I'll have the Bass i sprayed with me at the Bash and happy to talk you through it, mind there will be others there also who are much more knowledgeable than me on this too!