I made a couple boot blanks from polar fleece. I figured this would make a warm and soft interior and a decent framework for the rubberizing agent to stick to. I used HERCULINER Truck Bed Liner as my rubberizing agent. I'd been looking for a product that just had bags of granulated rubber and an adhesive separately, but this came with both mixed together in the can.
I stuffed the boot with plastic bags full of paper to get a boot like shape to start with, and started daubing on the rubberizing agent with a brush.
Several problems came up with the rubberizing process. The material is meant to be applied with a foam roller to a flat surface. It got very clumpy when trying to daub on with a brush on an irregular surface. Second I am not a neat person, and if you get a drip on the dry side of your boot or on the newspaper you're putting the boot down on, you're going to end up with paper adhered to the boot. Thirdly the adhesive soaked through the fleece and also adhered to the plastic bags inside the boot.
The next problem was that although I'd realized I'd need more room in the boot to get it on after stiffening, I didn't account for it enough. You can see the problem here in the trimmed down version of the boot. (I trimmed it at three different heights hoping to find one still usable.)
Finally, a trim that fits!
But the sole is lumpy, and the shape is a bit more rounded than it should be, leading to it being short in the toes.
You can see in this shot how little of the upper I laboriously hand sewed on is still present.
But it was not a total loss. I learned something about the fit of a barn boot, and didn't waste leather in the process. I also learned that the base material should be resistant to being soaked through so the rubber remains on the outside of the material. I may also consider rubberizing the toe area with the material flat before sewing up the heel.