You don't have to sand it all off but it should be roughed up a bit. I'd use either a Scotchbrite (scuff) pad or some fine grit paper. If you have a medium grit, that will work. Personally I'd try and use 150 grit or finer or you run the risk of adding scratches that will have to be worked out later. Then clean it well before you coat it again (resin, primer, bondo, paint....whatever. As long as it's roughed, any will work)
As far as covering the visor you can get away with regular masking tape (not the really cheap stuff, Scotch or 3m is best) if you don't leave it on for days. If you have to leave it on awhile I'd go with blue painters tape.
I'd like to see it when you're done.
Good luck!