BC, I've never tried it with plastic molds but I don't see why you couldn't. I've encased styrofoam coolers and fish containers and haven't had a problem. If the plastic is very smooth you may want to scratch it up with steel wool to help your mix stick better. Work on one side at a time and when you start to do the second side make sure the first mix is damp so the new mix will stick. One guy that use to be on this Forum (peakpoet) encased a wicker or wood chair. I don't know how it held up though. Somewhere online there are pics of a chair or sofa made from bales of straw encased in cement. Billie

Billie, thanks so much for answering. I'll give it a try.

I use plastic dish tubs from the 99 cent stores for
molds, I usually dump them out so that I can re-use
them. Also if you want large pots try the big toy
tub that has the rope handles.

Good Luck and Happy Gardening,
Gemfire