Rust convertor chemicals - which one ?

You'll open a can of worms with that question but here's my 10 pence if it's any help. I'm in the same boat as you with bare metalling a car underside.

Blasting is obviously the best choice however not practical for us mere mortals working out of our garage/driveways. So, depending on how heavily rusted the surface is and assuming it's still solid/savable, citric acid solution (i.e Deox Gel) is pretty good at getting the rust out of pitted metal. It is however a very slow process requiring multiple applications. I find it's not as effective as the powdered version however dunking an entire car in it may be slightly problematic . It is however the best practice to 'remove' rather than 'convert' rust.

I've also found the diluted Phosphoric acid-based solution such as Kleen-B from Rustbusters quite effective at cleaning out/removing rust from pitted metal.

Alternatively to that, you're into the convertor/coating methods such as Hydrate 80 you mention, and the likes of Jenolite FE-123 etc. I use Hydrate 80 but I'm on the fence about how effective it is.

Once you've removed/converted as much as possible, a good 2-part coating such as an epoxy mastic should seal the area.

As I say, you'll have a multitude of answers but that's my take on it at least. Remove rather than convert if you can - hope that helps.

ive heard about 'Rust Kutter' but its from over the pond. Ive seen it compared to other rust treatments and it seems to be the best rust killer/ cleaner before using your paint system.

I have tried hydrate 80 but I'm not convinced, used it on a rust patch and it was rusty again within a month, I then treated it with cortanin F which was still fine after another 4 months when I got round painting it, I use Cortanin F now.

Don't waste money on 'products' they are all just phosphoric acid in various forms. Go straight to the active ingredient.

that bare metal you wire-wheeled?

If it was rusty before, and is now shiny, go back and drag he edge of screwdriver across it and inspect the sides of the scratch you left?

It'll probably be orange . .. rust . . .

I paid a mobile blasting company to blast the underside of mine - took him longer to set up and pack away than to do it - and I did have to do a fair bit of sweeping . . . but for the clean surface that was perfectly keyed for protective coatings, it would have been cheap at twice the price.

You can get it in a gel form (Bilt Hamber Deox gel) which you can cover with clingfilm

Yeah cheers think ive heard of that too on recollection, come to think of it too the previous gels ive used looked pretty similar, would be fairly easy to create a concoction, with phosphoric acid tho would it dissolve the metal too if left on and not washed off or could you just use weaker solutions?