I've got a small puncture-about 4 mm-on the bottom of my board. I've filled the hole with resin, but I'm wondering: can I lay a small piece of fiberglass over the repair without[/b] sanding off the paint first? Or will it not bond properly?

Sand it or sand the paint off down to black stuff

Paint off. dont sand till you hit black you may not find it lol
it will be obvious when paint then undercoat /primer is gone and you hit filler coat / laminate

I agree but just filling a hole with resin it may not bond to the paint in the depression. So with no prep before fill, yeah now I'd glass it

Yes it works but not without considering heat. can melt the core

prob better off with UV cure resin to fill, then sand n glass. Still back on water next day

Or just use very slow epoxy - too keep the temperature of the chemical reaction low enough.

I've had a lot of success with Solarez for repairs like that, which have held up permanently.

I don't use Solarez according to the instructions though. No sanding back, etc. I clean the damaged area with IPA thoroughly, then put Solarez on directly in liquid form to fill the crack / depression in the board (with a cotton bud) and do this very gently and very neatly . Next - I take the board outside on a cloudy day and gently 'work' the Solarez to exactly match the shape of the surrounding board surface with my fingers as it sets. Solarez goes malleable and putty-like in daylight after about 20 secs or so - on a cloudy day. Better still, 'working' it this way gently with fingers actually leaves a matt finish when it finally sets, which is virtually invisible.

I know this is a completely unorthodox approach, but I've found this 'hybrid' method with Solarez really works for me and, importantly, avoids the need to sand back a massive area and do an ugly respray.

I've repaired multiple cracks like this up to ~5mm diameter or thinish ones ~30mm long, with absolutely no problems 1000s kms later. Some of these repairs are so invisible I haven't even bothered to touch them up, but if needed just a tiny dab of matched paint and a small brush does the job.