Pot melting occurs at temperatures above that for which kiln washes are designed. This means the kiln wash sticks to the back of the melt.
If you have a sandblaster, it is easy to take the kiln wash off leaving a matt surface. You can live with this for many purposes, but if you want a more polished surface you can take the melt up to fire polishing temperature. You will need to flip this over and fire again, if the original top surface is what you want to present.
If you are going to cut the pot melt up for other uses, there is no need to fire polish as the surface does not matter, only the cleanliness.
Another thing to do is to avoid the kiln wash contamination.
If you put only fiber paper – not Thinfire – at the bottom, the dripping glass will tear and move it about. The best solution appears to be to put a disk of glass on top of the fibre paper. It can be clear or any colour you wish. This seems to keep the fiber paper from tearing and being incorporated into the glass.
It also works very well when you are confining the melt to get a thicker disk. Make sure you have kiln washed the sides of the container or dam very well, or it will stick to the sides. You will need to grind the kiln wash off the edge of the disk, but this is much easier than grinding it off the bottom.