Mast Inspection Opinions

I started to say the reason for painting a mast was so that you can do it again in five years, but that's not really true.
When I owned a bicycle shop we frequently found severe corrosion where the stainless steel strap that holds the brake lever to the anodized aluminum handle bars. I remember one case where I pushed down on the bars and they broke away, held on by just a shard of aluminum. Look at a bike and you can see how sweat would accumulate at that juncture
creating a galvanic potential. I think this is exactly the situation you have with that SS collar around an AL mast.
Now, as to painting, when AL bike frames became very common, many had serious paint problems. As we now know paints that work on steel do not necessarily work on AL and sometimes large patches of paint will just fly off leaving a powdery aluminum oxide surface. Sometimes those riders, instead of painting their bikes, would polish them to a bright shinny finish which would last maybe a couple of weeks. I have not seen such a frame completely corrode through, but there were worrying indicators around welds, seat tubes and where parts were bolted to the frame. Today, painted AL bikes are very durable and last for years, but the paint process is very technical, probably not useful for painting a mast in the boatyard. So do some research on painting aluminum and follow the instructions and you should get a very satisfactory product.
Unfortunately, it's not going to be fast and simple. But then what is when a boat is kept in an environment like yours, and mine. The warm humid sea air rots everything and the best preventive measures only slow it down. A little.
John