Many opinions about using PB and many good results. You can mull with the "stomping" method but I think machine mulling is best and far less work, after you build a muller that is. Surprised how well DavidF's bucket & stirrer works but it does seem to work. If you shake out your mold too soon and there's lots of smoke that's oil you're loosing which will eventually need to be replaced. Patience is a virtue on that one. I scrape off the burnt stuff that I can easily, especially when casting higher temp metals. But the best thing I do is always face the mold with freshly mulled and sifted virgin PB then back it with the used mulled sand. Doesn't take much and gives pristine results.