Random orbital sander with a 100 grit pad would do a nice job of removing the gelcoat.
Anything more than four to six inches would be overkill.
It also depends on how many layers of glass you intend to put on... if you're doing two layers, four-to-six inches would be good...since you could taper the layers, having the first come two-to-three inches down, the next come four-to-six inches. If you were going to go with three layers, it might be better to do six-to-nine inches of overlap.
You can sand it lightly and then spray gelcoat using a Preval Sprayer. The Preval Sprayers
are small, disposable spray guns, which use compressed canisters as a propellant. They're only about $6 each.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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