What is the best way to waterproof plaster? What I heard so far is:

"Clear acrylic spray will do the job if you spray on enough, ie, beyond the
point where it just sinks into the plaster. This is the technique that is used
to seal cast plaster model kit parts, but with a lighter coat, so that paint
doesn't sink into the plaster like a sponge and spread laterally."

A few years ago I came across a product called Water Block. It worked very well on plaster.

There used to be a product that was a two part epoxy resin years ago but I haven't seen it lately. Try clear marine resin used on boats. It was designed for use outdoors in the sun and to resist moisture. Most home improvement stores sell it. Because it's glossy, you can spray a flat matte sealer over it after it dries to remove the shine. Good luck.

The very best waterproofing for plaster is Waterblok. It works great also on concrete, brick and other stone products. It goes under the paint, prevents the paint from peeling, this is a penetrating and not a surface sealer. Plaster statues will last for several years outside, comes with a five year garantee. It will also prevent frost damage in concrete. It is non toxic, great for sealing bird bathes. I can very highly recommend it, it is available in my store. Sells for $21 a quart and goes a long way.

Judy Statuary Place ( 714) 826-5240

i will have to rethink to buy this next time...

There are many different kind of waterproofer products for stone and concrete items available at places like Home Depot. They work well on plaster also. Just saturate it well before painting it and dry it on the sun. Do the inside , outside and bottom of statues. Be sure it dries thoroughly before exposing it to water. After painting it you can apply a sealer like you normally would, matte , satin or high gloss.

I found a tremendous product which we have been using for years.

Sheilds All 10 from Hytech... it was originally designed for the space shuttle.
It will completely weathterproof your product. but not "waterproof" which means your piece can not be in a submerged situation, sitting in a pond or fountian.

First apply our fancy finishes then coat the product with 3 coats inside and out.
Make sure you are not using a finish with glues like crackle because as the piece crackles more over time (which glues do) it will then make the item porous and not weatherproofed.