I've had pretty good luck with Spar Urethane outdoor. Be sure to get the water rather than oil based formulation for obvious reasons.
You can add pigment optionally. I also add tissue paper - like one would blow your nose with. This provides a structural framework that tightens as it hardens. I did a couple of layers this way on a stone that completely cracked and also glued the stone to a glass 3x8" blank so I could still use the stone in a stone holder.
You do want something like tissue paper that can be incorporated into the swarf in small amounts without concern rather than carbon fibers or fiberglass.
Most classically, one uses uroshi lacquer, however this is STRONGLY not recommended as it is HIGHLY toxic (essentially concentrated poison ivy). Among other warnings - use gloves, outdoors, etc, even with these recommendations, it is suggested strongly not to touch any membranes - nasal etc - including those of your significant other for at least 3 days. Pass.
Somewhat less classically but almost universally used in Japan these days is Cashew lacquer, however because it is flammable, it cannot be imported or mailed these days due to restrictions.
So for my uses what I first mentioned is what I use. I did this treatment on a Tajima stone broke in shipment. It's been working well since. I did go overboard with reinforcement, so it looks a bit rough around the stone
But while too rough to sell, I can use the stone for my own use. It's the one I used in the Tajima video with the burgundy coating around the stone.