I bought a bag of plaster of Paris at the store yesterday on a whim and decided to fill my Hatshaper plastic hat block with plaster. Why? Because my hatshaper is a straight sided dome, large size, and is a wonderful shape, even long oval like me, but about a size too large. I figure it would make the perfect mold for a plaster hat block why I could modify.
So first I searched my home for some kind of lubricant to coat the inner walls of the hatshaper. I couldn't find any Vaseline but I did have my trusty can of Murray's pomade which is mostly Petrolatum anyway (I got a case of this stuff, so I'm not hurting for pomade). Then I filled the hatshaper with plaster, but came up short because I, uh, ran out of plaster. But I'm getting another bag today.
This morning the plaster was hard but wouldn't come out of the hatshaper! Then I remembered that Murrays liquifies at high temperature, so I got out my heat gun, heated the hatshaper, let the Murray's liquifiy and thin out a bit, and eventually the block slid out!
Now I will add a wall around the bottom and add more length (yes, I'll turn it upside down first). While the plaster was still wet I inserted lots of toothpicks to the exposed surface at different angles to help the second layer of plaster stick firmly to the first. Unfortunately when the hat block slid out onto the floor, those toothpicks all got smashed.
Next I'll sand the whole thing.
My question is this: What the heck do I do after that? I have read that some people coat the whole thing with polyurethane. What is this for? Are there other choices for coatings? Is a coating necessary?