Don- In case you didn`t see my previous posts on this subject...
Do everything you can to grind out the visible rust. I use tiny diamond burrs on a rotary tool. THEN treat with the acidic stuff (FWIW, I like Rust Bullet`s Metal Blast...think that`s the name, my last fix has lasted so long I haven`t looked at the bottle for a decade
).
Use something other than POR15, there are better choices these days. My fave is KBS Coatings` RustSeal, which I find utterly superior.
The stuff Ketch mentioned is *GREAT*, but *IME* (think, "chance of user-error", not-Accumulator Proof, etc.) really needs to be applied with an undercoating gun at the correct (high) pressure unless you`re a lot better at stuff than Yours Truly. IF you`re using an aerosol product, I`d go with KBS`s Cavity Coater or *maybe* Eastwood`s Frame Coater (yeah, I know we`re talking other areas) as they worked perfectly for me whereas the aerosol version of the ValuGard stuff didn`t. (Again, that`s almost certainly user-error, but I`ve done this stuff since being schooled in rustproofing by a pro the `70s and I`m no dummy
).
And you and I are still on the exact same page about what`s "old" vs. what`s "merely getting broken-in"
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