Coming from a very sandy place called - wait for it - Sandy Point,

I have studdied this 'sand in the join' phenomenon for quite a few years. Well, since 2 piece masts appeared actually.

I have found that these things aggravate the sand filling the join the most :
1. Letting sand get in the top part of the mast (Doh!)

2. Not having a good mast plug in the top of the mast to seal it. Water flows downhill with gravity (now there is a revelation!

) If water gets into the mast tip it carries the sand down into the joint. This appears to me to be the main cause of the problem, but it can still get in there in the absence of water.
Preventions:
1. Using a mast with the ferule in the top half facing down. Not common, but some RDM's are like this and it really helps because of #2 above.
2. Keeping sand out of the top part of the mast (Double Doh!)

3. Having a mast tip plug. And then, a
good sealing mast tip plug.
4. Taping the join. This is not to stop sand getting in. This is to stop water getting in, and any water that gets in the mast flowing through back through the join and bringing sand in with it. Even if 2 and 3 are missing, 4 certainly helps a lot.

Just my observations.

PS. The same priciples apply to mast base extensions getting stuck in the mast. I try to
never, ever let the bottom end of the mast even touch the ground when I am rigging!

PPS. I am a duct tape guy. I find it sticks better.