I think these days most of the stuff which made paints work well are classed as toxic and have been taken out, the old fashion red red lead is not the same and in my experience hammerite is nothing like the product it used to be.
I have used several paints and found :
Hammerite. Not very good
POR. Very good but expensive and don't get it on your hands
Screwfix own metal paint Hard to apply not had enough time to judge the staying power
Rustoleum. Hard to apply not yet had enough time to judge staying power.
 
@aaronmorris galvanised his, and in retrospect I wish I'd done mine. But here's what I did do to my axles and all the other bits under the car:-

Two coats of Zinga from MGDuff immediately after sandblasting on a warm dry day. http://zinga-uk.com/product-range/zinga This gives a cathodic protection, and consequently doesn't mind being scratched right through. (Within reason).

Two coats of Zingaceram PU, as far as I can see it's like the Raptor stuff but with ceramics in to make it harder. http://zinga-uk.com/product-range/zingaceram-hs A bit tricky to get a gloss finish with a spray gun, but play with thinning slightly more than suggested.

Couple of topcoats of ordinary to match the rest of the car.
 
I have used a few corroless products and they seem good so far, Primer, Corroless S, and is nice to apply, quite high build and as such takes a while to dry properly.
Top coat corroless chassis in one, dries quick and gives a nice satin finish, but I find it hard to get a good finish with the brush. But maybe that's just me.. Sprayed is better if an option.
Corroless RF16 gives a better finish I find, is gloss and available in most colours.

I have also used Rustoleum combi colour on a good few projects over the years with some good results.
 
But yes, as Boguing says, I had my last couple of casings galvanised, and then painted in the above primer (corroless S) and top coat of Rustoleum, just in the hunt of trying to protect things as much as I can.
 

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