I found the Loctite (was called Permatex before) rust converter in the white bottle with orange lid to be good stuff. It's creamy, not runny so it can be used on vertical surfaces.
I painted the inside floor in mine with it 10 years ago, never painted over it and it's still good. It dries to a black paintable surface.
I used some POR15 on my car before painting it this time (around the 4 side seams and rear bumper seam), I'll let you know in 10 years how it's going...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
