Clean out the gutter, apply epoxy primer, then apply two part urethane self leveling seam sealer like this:
http://autobodystore.net/Merchant2/m...gory_Code=JASS
I put tie wraps in the slits that drain the gutter so the seam sealer wouldn't clog them. Once it set up, I pulled the tie wraps. Some folks fill the gutter to the top so the water runs over and can't settle there. The trick with the self leveling seam sealer is to squeeze it in during one pass. When you come back around it will have started to set up. Self leveling leaves the sealer as smooth as glass. Once it sets up you can paint over it.
Hope this helps.
Regards,
Steve
New Orleans