Koi Forum • Best way to secure/lay coping stones : Koi Talk
I used tile adhesive to secure my slate down as cement failed first time, then sealed inner edge with tons of polyflex hm. Polyflex HM is aquatic safe according to makers as they used to make LM but discontinued it so I spent ages trying to find alternative, finally contacted them direct and they sent out data sheet for HM which stated it was safe after cured. Far cheaper than anything else and my cappings are solid as a rock. I wanted water level right under capping stones which was another reason for sealing with polyflex, as I prefer that look.
Wouldn’t use more nails as don’t rate that stuff if surface is dusty you don’t get a good grab as it skins off when comes out the tube. If no where near the water level I would look towards different building adhesives there is a lot of PU based ones out there that grab far better than no nails.
I did mine with water and fish in pond just lowered levels for a day then filled up no problems with fish and no leaks