Hi Jim & Eddie,

Stainless steel rivets and aluminum have been used successfully for many decades in experimental aircraft designs that I know of. However, stainless steel can actually be quite far from aluminum on the galvanic corrosion chart... it depends on if it is "active" or "passive". But as has been said, it is critical to seal the contact area from moisture and/or prime the metal, traditionally with something like zinc chromate - an excellent protective coating for aluminum in contact with dissimilar metals - now considered a health hazard and difficult to get anymore. I have been in the habit of dipping stainless steel rivets in zinc chromate primer before inserting and pulling them on other airplane projects that I've done. Not only for corrosion protection, but to make them a little easier to drill out if it becomes necessary, without spinning in the hole.

I think that perhaps the marine industry uses a lot of micro balloons because of weight and especially ease of sanding or shaping. You're talking about applications where large quantities, percentage wise, of micro balloons are used. Flox filled epoxy can be quite tough and hard to sand. As far as electrically insulating two metal surfaces, the flox is also non-conductive (as is epoxy of course).

I don't think that the tiny amount of micro balloons specified in this application does much to make the epoxy a "good solid gap filling bed" any more than straight epoxy.