Water proof marine varnish on flagstone??

I cleaned my Pentair cartridge filter out for the first time last week and there was a lot of sand in the bottom of the housing. Since this was the first time I opened the filter housing since the pool was constructed I thought it might just be from the construction process. But then on second thought I have OKlahoma flagstone coping surrounding the pool and this is basically sandstone. In addition, I regularly see bits of the flagstone flaking off the edge into the pool. I am assuming the water is breaking down the flagstone and the flakes are turning into sand in the process. I see the same thing happen over time to the flagstone in the garden as it weathers.

My question is what if I used a marine waterproof varnish on the bottom edge and side edge (at the pools edge) to seal the flagstone. Would the varnish adhere and seal the flagstone and prevent flaking? Has anyone done this? I would not use the varnish on top as that would probably make it slippery.