It is a nice idea as well to put a clear piece of glass down on thin-fire and this helps the glass to flow nicely. If you prefer to use fiber paper;( I usually will use clean shelf primer myself, but that is just me).(I usually will saw the clear off if it is going to ruin my pattern when I re-invest after cutting ) or polish off if the piece is large for the saw, so the glass is nice and new and clean where the join will be if you are going to re-invest and lay those two pieces with the bottom touching each other. You just want to make sure the join is clean. (but then I seem to saw everything!!!