How best to forestall rust on exposed screw heads?

So it's been a year since our pool was installed. On our pool equipment on the side of the house, I've noticed that the screw heads on various valves that are exposed to the sun all day long have become a little weathered, a little rusty -- the screws on the valves under the roof eaves have no similar weathering.

I foresee a day when I need to unscrew those screws, but the screws that are exposed become so rusty as to make it very difficult to loosen them.

What is the best way -- short of replacing the screws with better quality stainless steel ones -- to keep the screw heads from rusting when they're exposed to the sun? Mind you, I'm not going to erect any kind of sun shade for those screws, so I'm thinking along the lines of something that I can apply to the metal to protect it better. Paint? Varnish?

I would replace them with stainless steel rather than wait for them to rust and then fight them.

Actually, Jandy says that they ARE stainless steel screws:

http://www.zodiacpoolsystems.com/pro...ds/SA5250.ashx

However, there's variations in stainless steel quality, and I'm guessing Jandy's supplier isn't using marine-quality stainless steel....more like cheap stainless steel used on inexpensive stainless steel grills (which rust after just a season or two).