^^^^^ this. Especially if you plugged the hole. On my fixed stem Toro if i plug the hole, it causes an air tight seal only letting water fill the tube up and not the joint. Even if the joint is filled make sure theres no air bubbles and it is completely submerged. My bet is it wasn't, as simple green took a month old stain in my treeless arm perc out, when nothing else would. These are scientific peices ( well idno about etsy) so they aren't as simple as one would think. ^^^^ this tooo!!
LegitDankz is right. You're not getting SG on the inside of the downstem because of the way you're plugging the joint. You need a real glass stopper. That way it fits pefectly and you can rotate the tube as you fill it to eliminate the air bubble in the joint so SG can fill it. I knew that downstem hadn't been touched. SG never leaves that much behind..... ever.
you used the wrong stuff. What you bought is diluted in a spray bottle. What you needed to buy was the 1 gallon concentrate bottle. Its meant to be mixed with water before use normally so its much stronger.
come on OP its obvious that the resin is on the inside of your down tube seeing is how you let the SG sit in your bong for over 24 hours and yet your bong looks great but your DT doesnt. Just pour a little bit of ISO into a saucer or really small bowl, then get some pipe cleaners, soak pipe cleaner in ISO for a second then scrub the inside of your down tube, problem solved.
the man has it right. soak it up and after pouring out the simple green/alcohol/oxy clean or whatever u want to use, scrub it with some q-tips and it will come right off, then proceed to rinse with water as usual