I am painting on stones for the garden with acrylic paints how should

Use non yellowing polyuerthane to seal them. Is that what you were asking? your question was cut off.

Polyuerethane can go flaky after 1w months of sun exposure and if the rock is porous will blister with water ingress. Only an oil based primer is water proof. Then house or exterior grade paint over for a lasting finish...

I live in the North East USA. I often use craft paint for outdoor accessories like animals / decorations etc. I then spray them with clear polyurethane. They last for several years. Not sure of they could make it too slippery to walk on, or if would wear from walking on. But if these are just for looks, shod be ok. I also put my better outside critters in the shed during the winter, to keep the snow off of them.

to seal them the only thing to use is a outdoor marine grade varnish.Polyuretheane will not work

I paint rocks with acrylic craft paint. When completely dry I spray with polyurethane finish. I give it at least 3 coats and it lasts for a couple of years.