"Walter R." wrote in message
...
My aluminum gutters are leaking where they are joined at corners. What
kind of caulk works best to fill and waterproof the joints, preferably
while they are wet.

Nothing much sets well when surface is wet, outside of roof repair goop.
Use either silicone sealant (not paintable kind), or polyurthane gutter and
downspout caulk. Both require dry and clean surfaces.