Speak to your local trade paint supplier, I use a Tractol VQD, you need a special thinner for it, paints about £30 for 5 litres, thinner about a tenner
Rustoleum also do a similar VQD range
We use one from the local motor factors, i think its an air drying epoxy, you need the right thinners for thinning it down, but standard thinner works okay as a gun wash/cleanup.
We normally only use RAL colours.
I use lowe rust primer you can spray it straight on rust but I like to give old gates a good run with a grinderbrush first. the paint im using the moment is an ordinary shed paint probably because I have 20 litres of it left over after painting a shed
I have used this paint before, it's very good for anti rust protection.
Look also at North Valley Forge (nvf) in Colne and also Shipley paint, for those in Yorkshire / Lancashire.
Any of these will send to you, loads of colours too.