Step 4: FLAME polish
Here is the tricky part where some skill is involved. The basic goal here is to essentially use the fire of the torch to melt the plastic, without burning the plastic. Burning plastic is characterized by bubbling and black specs. If you burn your plastic, sand off the damaged material and try again.
Place your plastic onto a non-flammable surface. I used a cleaned welding table. Pull the plastic out so the edge of it is hanging. This will keep it from melting a little bit and picking up the texture of the surface it is resting on. Turn on your torch. Since you are using a propane torch that produces a carbon monoxide gas, make sure you are using this tool in a well ventilated area.
You generally want to flame on your torch to be strong. You want to use the crisp,blue, inner flame to actually polish the plastic, but you should always be aware of the rest of the flame because of its ability to interact with your plastic. Now, with the lit torch, very quickly move over the areas you want to polish. This motion is almost like quickly brushing hair. You want to move fast so you do not burn the plastic. The plastic will retain heat energy, meaning that the more passes over the plastic you do with a torch, the more potential there is to burn the plastic. in addition, because of this energy being put into the plastic, there can be a tendancy of warpage, just clamp down the plastic (away from the areas you are polishing)
You will know when you have done the right thing with your plastic when it reaches its polished finish. I almost looks like a bead of water is running across your material when is it reaching the appropriate melting point. In the final image, i have a picture with what a polished area looks like (on the left) and an unpolished area (on the right). As always, it is best to test your skills first. Grab a piece of scrap where you can refine your skills.
You now have a piece of plastic with a finished edge.