Talking about sealing rivets, I don't like the little plastic bits you put in rivets to seal them. They have a tendency to fly away and they degrade quite quickly. On my Hobie (a long time ago) I used rivet casing to seal the fittings: you put the casing in the mast and pop the rivet into it. This has numerous advantages: no broken rivet head rolling in the mast and a tight fit that will not leak, especially if you dip the casing in silicone first.

At some point I had monel rivet casing, and I've seen aluminum ones. But I can't find those anywhere anymore. There are only SS casing these days, in only one dimension and I don't know where to get them in the UK anyway...

(If you have no idea what I'm talking about you can see them page 14 of the Murray's catalog)