[RichH]

Diesel residue at the exhaust port is a combination of two actions due to the ''porosity'' of gelcoat. This means that if one routinely seals the gelcoat surface with wax the cosmetic problems of black unburned carbon and the staining due to rust will not occur (as often). Once gelcoat becomes oxidized it also becomes porous (ultimately it will become ''alligotored''). So, no matter what you use to clean this be sure to wax the area after cleaning and you wont have such a problem as often. Colllinite "Fleet Wax'' seems to be the longest lasting wax in the NE USA.

Agressive caustic cleaners such as those contain sodium silicate will typically work best for the black staining due to attached carbon particles. Once the carabon migrated deep down into the porous gelcoat, you will have to sand/abrade away the upmost layer of gel to restore color. Rust is best removed by application of oxalic acid; oxalic acid obtained from a hardware or paint store (cheapest).

Oxalic is probably the best to remove the brown tannin, etc. stains on waterlines and bow (and engine exhaust ports) .... but if such cleaning/bleaching is needed, it really means that the gelcoat has become porous due to oxidation and should be sealed regularly with wax.
:-)