I am using acrylic pain to paint on stones...

There's a Mod Podge that's made for outside use (I think exterior may be in the name). Or a can of polycrylic sealer. I'd do 2-3 coats.

I would apply a light coat of two of poly.

I would use a nonyellowing polyurethane. I'm not sure modpodge would hold up. Make sure to do several coats.

Hi

Exterior Grade house paint is best. Actylic or water based. It will breakdown over time but looks goid and weathers ok. Check out my fairy garden concrete mushrooms idea...

Modge Podge has a formula for outdoors but I’ve found that the best way to seal so it lasts is by spraying with polyurethane finish.

A few coats of clear coat either brush on or spray should protect them for a long while.