what i always do when i go windsurfing at sea:
when i take out my mast, i clean the parths that need to be joined with my hand so all the sand is off from both joining sides. I put the mast together, turn it a few times in both directions and take the mast appart again to see if it still has sand somewhere, clean it again with my hand and put it back together. After that i quickly, without putting the mast down take some ducktape around the joint so nothing can get it.
Also when you can't get the mast appart sometimes it's because of salt (but this should only be in warm places where the water had a bigger chance of evaporating). The thing i found worked was to get a few strong ppl (you have no idea how big some saltcristals can get) and but the joint under a fresh water tap and let the water run (i know, it's spilling, so sue me) whilst you try to twist the mast in opposite directions (one half clockwise, the other anticlockwise). Once you can twist the mast, you can pull it appart.
hope it helps!
cheers and happy NY!
Haiko, AKA crazychemical