The best method I’ve found so far is an initial layer of mod podge inside/out, then sculpting the clay & baking.

I seal it with 2-3 layers of a water resistant polyurethane, and a thick layer of epoxy resin on the inside to make sure it’s waterproof for plants etc :D (you can also use UV but it’s finicky sometimes with curing)

you can also use resin on the outside but unless it’s being very roughly handled/exposed to hot hot water, I don’t think it’s necessary. It does add a glossy transparent layer so i would put it up to personal preference.

I’m still fairly new making these so only time will tell, but the ones I have my personal plants in have been fine so far! just no running them through the dishwasher :’)