I've got a couple of deep Tuttle fins that are very slightly loose in the box, what's the best way to build them up. Was think8ng a few layers of paint
Resin best (after a good rough up and clean)
The problem with just a little bit is it flakes off when so thin.
Paint doesn't set anywhere near hard enough unless you use a good 2K and even then that's softer and more expensive so just go resin.
Probably going to get red thumbs, a strip of duct tape is a good temporary fix for a slightly loose fitting fin
Tuttle box heads tighten on the end taper. In my experience it does not matter if the sides are a slightly loose fit, as long as the ends pull tight when the base is flush with the bottom of the board. If the end pull tight, the fin really cant move side to side.
You can adjust this by shimming the ends (just the end surface) with adhesive tape or by building up with layers of resin. I often use thin plastic strips cut from butter container lids and just sit them in as I put the fin in. 
Select to expand quoteMark _australia said..
Resin best (after a good rough up and clean)
The problem with just a little bit is it flakes off when so thin.
Paint doesn't set anywhere near hard enough unless you use a good 2K and even then that's softer and more expensive so just go resin.
That's what I thought about epoxy which is why I thought paint might work, might just leave it as it snug when done up it just annoys me that there is a bit of a gap in the middle. The front and back sides are tight
You can use "sandable primer" from a spray can for small gaps, and can apply several coats if need be, as long as you let them dry fully and sand a little between coats. The treatment won't last forever but is easy to renew and you generally won't apply it as unevenly as you might with putty.
it's not strong enough for the end tapers where the real load is, but for instance can give you a snug enough fit to turn the board over without the fin falling out.
Also great for those hair line scratches you get on your fin from the sand.