That "powder" sounds remarkably like corrosion, so it might behhove you
to drill out a couple of the rivets with the most powder and see what
lies beneath.


Once you have eliminated corrosion as the source of the powder and repaired as
needed, try some Superfil. I've not heard from anyone that was dissatisfied
(knowing this group that may change). I'd stay away from polyester based
filler. It tends to swell in the wet and crack in the cold.