The obstacle is not how to coat the inside of the frame, but how to clean it well enough for anything to adhere. Here in the Northeast, the only Bronco frames that survived in great shape were oil undercoated annually. Now, when doing a frame-off restoration, the frames are a filthy mess. I spent the better part of 6 months cleaning my frame before media blasting and painting the outside surfaces. The best that can be achieved internally, is to use a stiff tube/chimney brush to dislodge 40 years of crud. Before putting the rig back on the road, will likely spray the inside of the frame with Fluid Film to preserve it for the next 40 years.