I had some wire trees I had added hydrocal plaster too , for bark. It was smooth and lumpy and set faster than I was anticipating. It Looked poor. I let it go for a few months, but finally fixed the bark situation. I was cleaning my table saw bag, remembered how I used sawdust on my railroad a few times as I did. I decided to try it as bark on my trees, a fine pine sawdust.
I made a glue sawdust paste applied it over the hydrocal failure and after dry painted..I liked it so did a couple others, I'll probably work it into all my future trees... here is a view.
sawdust bark

This is really quite good!! And to think of all the sawdust I've cleaned up and thrown out after home improvement jobs!