Its not hard to do, but requires access to some fairly special tools, ideally an oxy-propane or oxy-acetylene torch.Here, I am using a "Jewellers" scale torch which seems to be fine for my size of work.
Polishing works by locally melting (NOT burning) the surface, and the low viscosity of the melt and surface tension in the liquid plastic does the rest.
Step 1: Preparation.
Next, I take a "naked" Stanley knife blade, and use it as a scraper, to plane off the protruberances.
Step 2: Scraping
The noise isn't pleasant, but the work gets the job done quickly. You can see in the picture, the snow like planings coming off the work.