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Join Date: Sep 2005
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It depends on what paint you are planning to use and what paint is already there. If the paints are the same type and are known to be compatable, then a thorough wash (get rid of all slime) and a quick scuff sand should be fine.
If you don't know for sure what's underneath, it may be best to remove all old paint. Worst case scenario is your new paint could fall off after basically dissolving the old.
If you have several seasons of accumualtion you may want to get rid of it to avoid having too much paint there.
With ablatives it's a neat idea to have the first coat in one colour, and subsequent coats in a different color. That way when you are wearing thin, the original coat is exposed before you get through all the effective paint, and you know it's time to redo.