The soap actualy lubricates the paper and it will sand smoother. As far as finish goes I dont think it will make a difference. I have heard it makes the paper last longer though? In any case the main thing is to keep your area saturated. On a side note. I cannot find on the net anywhere where tremclad makes rustoleum and that it is in fact the same product. I was thinking about using valspar tractor and implement enamel from TSC. Any advice or info on that would be great. I was worried about it though because it takes a hardner?