The rust is from the pot oxidizing - sounds like you have a lot. If you drain your pot you get more oxidation on the sides of the pot. I leave about 3/4 full. If you leave too much - the lead doesn't heat up evenly when you go to repour and if your spout isn't fully seated you're going to have a big leak problem. With the lead & problems you've had - maybe flux more often. I use a long handled tea spoon to add wax and scrape the sides of the pot & stem while fluxing - the rust doesn't automatically rise to the top. Be careful and use gloves.