How long do you leave it on?
I have a bunch of crusty stuff on the lower half that is going to take something strong to get off.
If you have a larger toon such as a 24' like I did. Take it to a polish shop for semi's. They charge very little to put an acid mixture to it, and clean it free of all debres. They can also put a polish on the toons, that is second to none.
They use the same compound that is used on fuel tanks for semis.