...depends on what you want to achieve. In the case of that Lenton, in the few spots that were rusted back near the dropouts, I used some Rustoleum rust converter/primer tha leaves the spot black and ready to accept paint. Anything loose orr peeling needs to come off, but otherwise it's pretty user friendly. Note that I am not using this stuff in accordance with the commonly accepted "recoat window" advice. I accept all that as probably the best way to achieve maximum advantage in your final result (in terms of toughness and durability). But there are some practical limitations when painting bicycles that get in the way. One of them is application of stickers, which is kind of an ifffy experience when your color coat is less than 24 hours old. Another is in using the stuff as a topcoat in an attempt to preserve original paint and decals.

I freely admit I'm using it in experiments, and my experiments are in contravention of the literature that comes from the manufacturer. But I'm thus far pleased with the results.