It's because the stuff is so VERY gentle. If it has a finish - it just needs a little cleaning.
Out on deck with the gelcoat you may need something stronger. Here we have a coal power plant and an oil refinery near by and the two make a cocktail that is not so easy to get off.
Try a Vinegar/Bleach mix on your grp with a sponge pan scrubber... just make sure you wear a mask with filter or it'll knock you out and rot your lungs... strong fumes....
Had some red dust settle on the boat while on the hard in Portimao, seemed to eat right in and normal products could not shift it.. that did.. but wash down well after and put some wax back on the smooth bits as it strips all the protection off...
“I do not exist to impress the world.
I exist to live my life in a way that will make me happy. ”
Initial application of a few, 4-5, coats of tung oil. The more coats, the more shine. Then maintenance with Murphy's oil soap to clean, and Lemon oil to preserve. After 26 years it still causes folks to comment..The main thing you want to avoid is letting the wood dry out. It's like human skin in that it likes to have a certain amount of moisture in the surface.
If you've got "wood" veneer or otherwise, and is protected with a finish, lysol products sprayed on then wiped will remove the dirt and old oils as well as put a stop to mildew growth. Then Murphy's Oil Soap or other hardwood cleaner followed up with a protective coating ie wax or oil.