Look out, big head has arrived.
Sorry to tell you, but you may not have been successful. Leave the aluminium open. The more is it open to oxygen the thicker the protective layer, albeit slowly, will develop. Aluminium oxidises instantly in contact with oxygen(air). The paint will not adhere properly and Hammerite in particular will not stay there for long. If fact, it used to say on the Hammetite tin that it is not suitable for use on aluminium. You can paint aluminium with etching primer, which is designed to improve the adhesion, followed by top coat. Perhaps that is what Charlie has done, in which case, just ignore me.
Removing stubborn screws.... the aluminium is sacrificial to the stainless screws so corrosion around them will be at its greatest. Get a good fitting pozidrive screwdriver, hold the screwdriver in your left hand as though it's a dagger (presuming you are right handed), place it into the screw head and whilst undoing the screw hammer the end of the screwdriver to apply a shock load to the screw. Of course, if you've previously chewed up the head it'll be more difficult, in which case, hit the screwdriver into the head before attempting to undo the screw by the obove method.
It's just a crude version of an impact driver, but it works.