If it's fine enough to sneak past everything you throw at it, a wrap or two of tape should do the trick.
Actually, I don't have a problem (anymore), since I began using the silicon tubing, on all six joints. I must be using that soft "gummy" stuff, that Pete referred to. Either way, it is very pliable, and it seals up the crack completely, between the end of the cap, and whatever tube I'm using. I just don't get "any" sand, in the end caps, now.
As I mentioned previously:
"I fly predominately on sand, and the surgical tubing has worked flawlessly, ever since I started using it. Also, I use the tubing at all six (6) junctions - both ends of the leading edge, and both the top and bottom, of both verticals. Yes, the upper verticals already have factory supplied dust covers, but fine sand can still get in, especially with 2-wraps & race rods. Again, I've never had a grain of sand, to get into any joint, using the surgical tubing (silicon tubing with a 3/8" OD x 3/16" ID, cut into 1/4" lengths), and now I have it on each end, of each frame section, that goes into an end-cap, and I see no reason to remove it."
I know it's hard to believe, but you just can't imagine, how fine the sand on these dunes has become, over the years. The sand on the beaches, is generally, much coarser, and not as much of a problem, so normal precautions work fine there.