Have you ever tried using AEROKROIL? It is the best rust buster I have ever used. It makes PB Blaster seem like water. It is not available in stores though. You have to order directly from the company or you may be able to find it at a industrial supply vendor. They use it around the power plants, chemical plants, refineries, etc. Just make sure to have plenty of ventilation. It has some really strong fumes. Probably not exactly good to be breathing a lot of it in. But it works great on rusty parts.