If you're doing the repair on all the bases, you might like to consider improving them by making a small plinth so that rainwater no longer gathers around the mounting bolts holes - it reduces the leakage tremendously. It's likely that the crazing in the gelcoat is simply star crazing from flexing when people lean on the stanchions and doesn't indicate damage in the main thickness of the lay-up. Increasing the size of the backing plate and building up a plinth with epoxy should make an effective repair.