I've been wondering if this is safe to do, since the photos I've seen of the heater tubes seem to show that they're corrugated paper on the outside with some sort of thick insulation layer under that, all around a corrugated aluminum inner tube. Seems like the conversion chemicals might attack, or at least soak into the outer covering and insulation layers and cause problems. My longs are in pretty good shape, only very little rust on the bottom of the inside, and I'd like to keep it that way. But I also want to preserve the heater function. What say the experienced rust-warriors among you?

I've never head of this being a problem.

I have used it in the longs and it dries very quickly. I doubt it will attack the aluminum.

Outside of the heater tubes are made of paper/cardboard. That said, I don't think there would be a problem, as they're pretty thick and I think they are coated to protect them from moisture penetrating. The factory got overspray on the tubes, as they put the asphalt covers on the inside of the longs after paint.

No issue with using it. It goings in very fluid to help the solids flow into the seams but the solvent evaporates fairly quickly. It will stay soft for a time while the chemicals work on any surface rust. It is a self etching product. It dried hard in a day or so. I have used it and see no issue.

Thanks for the advice, Guys. I'll go ahead with it.

Works great in the tunnel area too