-Leave them bare in order to mute the corrosion that becomes an issue with the first inevitable scratch...
I'm guessing with a 4 year old AI your corrosion looks like this?:
It has nothing to do with scratches. It is the result of a faulty e-coating process that was a real problem with 08 and earlier AI's. These are the aka from my 08 AI. Note how the coating bubbles. It's as if the coating itself is trapping moisture.
BTW -The 'new' looking brace was fitted in Feb 09 (replacing the 08 brace lost overboard after a collision) and it shows no sign of this corrosion.
The entire hull (including crossbars) was replaced with an 09 after a drivewell crack. The 09 e-coating process was much improved. In this pic you can see scratches from use but very little bubbling corrosion.
Compare the above with this pic of my 07 small sail mast which is anodised. Still looks like new today yet it has seen much use.
I believe the newer aka and crossbars are now anodised.
To me it's just a cosmetic issue. It looks ugly but I don't recall reading of failures caused by the corrosion. The tramps cover up most of it anyway! Some members have gone to the trouble of sanding back and repainting but report that the bubbling begins again, which is why I've just left mine as is.
Remember if you sandblast or powder coat you will have to remove all the riveted fittings.