I have a double u perc HiSi stright tube, with hisi ashcatcher for it. I've come to notice that only the top perc gets dirty when smoking out of it (Resin Buildup). The ashcatcher, and downstem build up alot of resin quickly, but i get no resin buildup in the actual bong until it passes into the second perc. Is there a reason only a top perc would get dirty? anything under it stays pretty spotless which confuses me.