The folks that make Nicro Vents also sell a white resurfacing spray promoted as "...chemically bonds to the surface forming a flexible, non-cracking, like-new finish."

I used this on my 1979 sunburned and discolored rubrail and it made it look new. I was all set to tear the whole thing off and order some kind of new extrusion but someone posted this solution either here or on the Yahoo board. I purchased a single can from West Marine (product link below) and it did the whole rail.

The first time out I picked up a nice black mark from a tire dock bumper. I used acetone on the mark and it took off the black but the acetone also removed some of the new coating kind of acted like a "liquid sandpaper" ... so much "for chemical bonding (next time I will start with a mild cleanser). In anticipation of this possibility I purchased a second can of Nicro-Cote and it will be simple to mask off and touch up the area.

Not a perfect solution, but beats the heck out of replacing the whole rubrail and then being right back were you started anyhow.