Yakima: No resin or acid core solder directly into the pot. I did have a lot of that stuff and smelted it outside with an old pot (not the one I am currently using for casting). It was pretty ugly, but after the melt was cleaned up, it works well. I cast it into 0.690" round balls to make it easy to add to the mixture. When I run out of the solder, I have about 15lbs of solid 50-50 solder bars and after that, a few pounds of pure tin. I will undoubtedly be casting them into balls, too.
Rick: My experience with Marvelux and boric acid is that they are quite a bit different in action, but both fall short of what I am looking for. The Marvelux worked much like beeswax for fluxing, but without any smoke. It was easy to break up and scoop out when done fluxing. The boric acid does nothing for fluxing. It starts as a powder, but melts into a form of glass to protect the top of the melt. It is easy to stick something into the melt, twist it, and take out most of the molten glass when it is time to recharge the pot.
However, where the both are the same are when they stick to the sides (and both of them do), building up layer on top of layer of insulation. Not good for running a hot pot. I have removed both from the inside of the pot with a coarse file, but it is not nice work.