Honestly, & this may hurt, but you really need to pull the stick, & do both a full inspection
of the mast itself (especially the butt), & the mast step.
Or it's what I would do, were it me.
I say as much, as to some degree, there's going to be corrosion
, & who knows how much physical degradation to the integrity of the base (& perhaps other internal areas) of the mast tube itself. Ditto on your mast step. Depending upon how long this has been going on, what materials are involved. And, plus, if there's any stray electrical
currents in the bilge water
or the mast tube, from instruments or lights etc.
The damage inside of the mast & step could be primarily cosmetic. And only require some removal
of the corrosion
in the area, prior to redoing the finish on the butt of the tube. Or you might need to bob the butt a bit, & have some work done on the step.
Some of the degree of corrosion can be seen via a camera
, & some can likely only be seen with the mast removed. Generally speaking.
And of course without having seen the video, personally, I (& most others) can at best, only speculate as to both the problem(s) & their cures. Although I think that some of the "sand" & other grit like material which you're seeing, is likely aluminum
oxide, which has flaked off from the insides of the lower sections of the mast tube & possibly the mast base.
If you did want to, or were in a situation where you HAD to seal things (up). Then you could do a fix like they did during one of the Volvo
Races in the '90's. Where they sealed up all of the openings in the (aluminum) mast tubes, which were having cracking problems, due to huge loadings on the spars. And then they poured epoxy
resin down into the tubes, in the sections of the spar belowdecks, where the cracks were occurring.
Having plugged off the appropriate internal areas of the mast, post an engineering convo & assessment with the shore team (& the expert riggers onboard).
They had go go slowly, only pouring 1/4" or so at a time (vertically), so that the resin didn't go exothermic on them. And after X hours, they'd pour in another 1/4" of resin. While the previous pour was still green, but mostly cured.
All while trying like crazy to keep the water
out of a mast with all internal halyards, while racing
full tilt in the Southern Ocean.
It worked, from the standpoint of stopping the structural cracking from spreading. But of course, was a nightmare for the crews to sort out, when the boats made their next port call.
Especially as, with a tube that's watertight like that, even with small holes drilled into it for drainage, above the seal, the metal down where the seal is, is continually immersed, with only a layer of anodizing for protection.
Unlike what goes onto an aluminum hull
Although, some of the major spar makers do have setups designed so that you can internally seal up a mast from vertical water drainage, just above deck
level. And so that the mast is protected internally from water damage. But that's something probably best discussed with them directly.
As, with hollow aluminum masts, even with all external halyards, you're still going to get decent quantities of water inside via multiple other access points.