If you don't apply heat evenly to the bend line of a sheet of plexiglass or acrylic you can create bubbles or scorch marks which cannot be removed. Do not keep your heat gun or torch too close to the plastic. If you are singing the wood on your jig (one reason why I use wood) you are too close! If you find bubbles or scorch marks occurring in your test bends, try to move your heat a bit faster along your bend line, or hold it further away from the surface of your sheet material. The distance and speed will be different depending on the thickness of the material you are working with.