I had laid down a thin layer of sand on top of my wet expansion joint goop, a couple days ago. I vacuumed off what didn't stick today. The goop had dipped a bit here and there, but for the most part the joint looks great. And will certainly do its job for a while.
I used Sika Pro Select Self-Leveling Sealant. I think there are better choices, based on what I read here, but Sika is a one-step (no mixing!) caulking tube application that is very fast and easy. I didn't mask anything, the caulking gun was easy enough to control. And the Sika wiped up easily with a wet rag in the few spots I spilled over. If the sealant started draining into some bottomless pit, I shoved the tip of a tube of painter's caulk into the cave-in and stopped it up quick. I'm not sure that's particularly sanctioned by Sika, but they said I could use caulk to plug up holes (like backer rod). I don't think they meant while the Sika was already applied, but that's what I did... so be it. The sanded finish looks very much like the grout seams elsewhere in my deck.
I can cross this one off the list, and I'm very happy to have this chore behind me. Next up: clean my filter and deal with the calcium on my edge tile.