You could also buy a special "etch primer" to paint galvanise, although unless the boiler looks very bad I wouldn't bother, it'll heat up with the sun and use (obviously) and I wonder how long a conventional oil paint will last. Don't use emulsion in anycase. If there are just a few rust spots you can buy an aerosol of (CFC Free) cold galvanising spray, it'll match the existing metal finish and protect it from any further rust. Most car/ motor factors stock this spray, costs about €20 but really will stop metal rusting.
For painting PVC soil pipe etc. manufacturers recommend you roughen the pipe surface with sandpaper and apply two coats of gloss, no undercoat required.