I wanted to spiffy up the 18 a bit as I have been doing a few things to it so I ran by boaters world in search of some good hull cleaner. I got what the guy in there reccomended (not that he acted like he knew what he was doing) and took it home. The stuff I got is called "On and Off" by MaryKate.
http://www.boatersworld.com/product/179010038.htm
Anyways, the instructions say to brush it on with a paint brush and spray off 60 seconds later with water hose. I was very skeptical to say the least. Well I started trying it on a few rust spots from the deck scupers, bilge outlet, etc.... I was AMAZED to say the least!! This is some fine stuff! I finally discovered a better way of application though. I put it in a spray bottle and sprayed an area down, then ran over it with a soft bristled car brush. This was by far faster and removed more crud. This stuff got rid on waterline stains, rust stains,...EVERYTHING!! It didnt hurt my boot stripe or my ID lettering either. I did the whole boat inside and out and she looks like a new shiny dime! It really put alot of luster back into a 16 year old gelcoat. 8)
CAUTION: THIS STUFF IS ACID!! WEAR LATEX OR NITRILE GLOVES AND
DO NOT TRY AND SMELL IT TO SEE IF ITS STRONG ENOUGH! I AM MISSING AN EYEBROW AND ALL NOSTRIL HAIRS FROM DOING SO. ::)
Also: Use in well ventilated area....like outside, do not do in a garage, you will die. :o