What I found is going over them lightly with a stainless brillo ball, like the type you would use on your grill. go lightly on the dried deposit and it comes right off then finish with polishing compound or Mothers....works good

I shy away from any abrasive, no matter how fine. It can "open up" the finish with fine scratches that dull and trap more residue. BioKleen oxidation remover or hull cleaner work great without the vinegar smell or harsh properties of acids or chemicals.

Muratic Acid, Sno Bowl or The Works toilet bowl cleaner, CLR or Lime Away. All will work equally well.

Rinse very thoroughly when done or you will get some light surface rust.

Your problem is you dont run aground often enough... hit a few sand bars a little more often, and you wont have that pesky problem...rm