Re: sandblasting TR4 chassis won't remove rust ins
Hi Phillipe,
It is possible to have a frame "dipped" to strip old paint and rust away. However, the chemicals get into every crevice and are hard to completely remove, might cause more problems later by seeping out and damaging the new finish after the frame is back under the car. The chemicals might "thin" the metal too (an old racer trick to lighten their cars), and can leave the now-bare metal inside even more vulnerable to new rust!
Short of dipping, there really is no way to completely eliminate rust from the inside of the frame. Just have is sandblasted and check that no areas are too thin and weak, might need some repair.
With that done and the outside refinished to your liking, the inside should be treated with something like Waxoyl, as 71tr says. That product is sprayed inside the frame through 1/2" holes. It's a mist and settles on all surfaces, sealing the surface. The holes are then closed up with plugs. It should be re-done every few years.
The point about being sure all drain and evaporation holes are clear is a very good one, too. Internal frame rust is probably 80% from condensation, 20% from water intrusion. Air circulation to allow it to dry is important and will go a long way to preventing or slowing rust.
Any accumulated mud or dirt buildup inside the frame should be cleaned out too. Underneath this is where rust really gets going, when moisture is trapped.
Finally, one place to look at very carefully is the diagonal brace that goes from the shock/spring tower to the main frame rail. These did not have a drain hole drilled by the factory and are very susceptible to internal rust. If your car is all apart for frame work is a great time to drill a 1/4" hole on the inside of this brace (toward the engine), just above the weld at the main frame rail. I'll bet if you do that a bunch of powdery rust comes out! If not allowed to drain, it's common for this brace to eventually fail, rusting from the inside out. In extreme cases, the rust can extend to and destroy the main frame rail immediately adjacent, too.
Alan