Here is my process. It may be overkill but like I said it's works for me. First thing is I've never had any luck with Mod Podge but others have so try your luck with it if you wish. First I'll remove the glue from the stamp. If it's the old lick and stick I soak it in water for a minute or two and use a very soft brush to wipe it off. If I t's the peal and stick, Forever Stamps in the US, I'll soak them in white gas or Coleman Lantern Fuel for a minute and use an old credit card to squeegee off the glue. I use a brush on water base acrylic paint to paint the tube and the back of the stamp. I use white for the stamp to keep the resin from soaking through and discoloring the face. When that is dry I use Elmers to glue the stamp to the tube. I'll let that dry enough to keep it from sliding and then coat the entire tube and stamp with the Elmers. That needs to be completely dry or it will cause pro twitch the resin. Good luck and feel free to reach out and ask more questions.
I followed your directions Kenny, and so far I am very pleased with the results. I removed the glue from the stamps, painted with 2 coats of white acrylic paint, adhered to the tube with Elmer's clear glue, and once dry, sealed the outside surface with 2 coats of the same Elmer's clear glue, making sure to cover the edges of the stamps and the ends of the tubes. These were cast in Alumilite slow inside a pressure pot. I have not turned yet, but I do not see any yellowing or bleed thru. Thanks for your suggestions!
I'm now trying the same method on images printed on plain computer paper, in lieu of using labels.