We've had the same problem with our PVC dorades. We replaced the old ones with new from West Marine
. When they in turn started having the same problem WM replaced them under warranty. Soon though they were showing the same problem, sticky to the touch and grungy looking. The problem, I think, is from all the pollutants in the air. I say this because the problem is at it's very worst when we are in a large metropolitan area. So far the Hampton Roads
, VA area with it's multitude of civilian and military airports, coal fired power-plants and, of course, all the traffic has been the worst.
The best we've managed at this point is to simply clean them with Awesome Orange then coat them with some "303 protectant". So far the process has brought them back to life, and lasts for variable amounts of time.