Bilt Hamber sell a product. I don’t remember the name but it’s a blue milky product. You paint it on and it dries black converting the rust.
You then paint over with zinc phosphate.
Hydrate 80. Same kind of stuff. "rust convertor" The Navy Steel is supposed to be the best one but who knows. Better to get the rust off rather than convert it I think
I've used BH Hydrate 80 on my car restoration. It works very well but I would only use it on surface rust where I can't reach to remove it with a power file. As rust converters go I would say it is better than the stuff you buy in Halfords. The worst thing is if you don't clean the thread on the plastic lid, it is near impossible to get the top off the bottle next time.
Loads of steel boat owners use it because its just its the next must have thing advertised in boating magazines, the fact that they removed the rust before using it doesn't occur to them that they are completely wasting their time and money.