I recently snagged some more pipes to restore, but these have been sitting for quite some time in someones attic. As a result, the outside of the bowls are quite dirty. I have tried a plain paper towel and even a q tip with some saliva to get the dirt out, but it hasnt been that simple. Anyone know of a way I can get this dirt off? Im afraid to use any kind of chemical or abrasive as I dont want to harm the outside of the bowl.