What do you reccomend to remove bad barnacle build up on 50 ft fiber glass? Just water and pressure?
Not unless you use a lot of pressure and a 0 degree tip. The drive up car washes use about 1,200 psi and lot of Corvette owners use them. A 50' boat has a very thick hull and the gel coat is designed to withstand a PW.
I've PW tons of boats but none with barnacles.
I have a friend in the Keys that owned a boat dealership. He said they ether have divers do it (really expensive) or dry dock them for a few weeks to dry. Then PW the shells off, scrape and finally acid wash. Then apply good bottom paint. A lot of work no matter how you look at it.