Is my shock past it?

Recently my air shock has developed a shiny white hue on the top third or so of its travel. It's also slightly rough to the touch. Ive been using it regularly for about 18 months but as this is my first full suss bike ive no idea how long shocks are meant to last. Any help with what this problem is and how it can be solvedwould be greatly apreciated (if, as i suspect, i have to buy a new shock i may as well buy a new hardtail frame because i already have a fairly major dent in the top tube. Thanks.
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sounds like the finish has gone.
but if you have not had oil every where i guess it is not oil damped. so it will not make much difference.
Parktools :?:SheldonBrown