Bung: That looks all superficial, I'd wire brush either by hand or with a drill the worst of it and spray with one of the cold galvanising zinc sprays. If you bought a tin to hand brush it, the left over paint probably would be solid before you could use it up.

Edit: Take a look at the rear tailboard hinge. The weld looks cracked, maybe it hit something.

The left-hand hinge completely fell off so I will need to replace one hinge and repair the one you see here. I'm wondering it is worthwhile treating the rust with rust converter before applying the zinc spray?

Dulouz:

 

I'm wondering it is worthwhile treating the rust with rust converter before applying the zinc spray?

 

In my opinion I would definitely apply rust converter first then zinc spray. Remember rust converter needs rust to react with to work, so only remove any loose rust flakes but leave the other rust behind to react with the converter.