I’m still intrigued with the idea of Burlap as a skin...

Cheap enough for any budget!!!

Free if you use “sacks” !!! If you want STRONGER, double it up ... “two skins are better than one “ ..

OK, it does have a large weave, but that could be filled in another manner... something we’ve not discussed as yet (as I can remember)
Acrylic Modeling Paste ...

Acrylic, so is paint compatible , Flexible , perfect for our use, very “FILLING” ... used as a "skim coat" , with a little practice , it’e easy to achieve a very smooth finish ... You don’t need much because you’re not building up a finish (sculpting , it’s intended purpose) , but merely filling in the weave... It does take a little practice to get it “just right” ...or just put on another coat ... it sands well too ...

I’ve used this preparing canvas for painting , some of which was a very coarse weave , much akin to a sack ... ( I remember seeing a Van Gogh at the Met that was painted on a sack ... True !!!)
The test : Try tearing a Burlap sack with your hands... ain’t gonna’ happen !!!