I'm pretty sure the "paint" on the back sie is gelcoat too unless you have some custom paint job over the gel.
Only way to get oxidation gone for good is to buff/polish it out with a rotary polisher, not orbital. Even a high speed 3/8 drill motor will work unless you're doing a large area.
I'd reccomend 3M Rubbing compound and 3M Finesse It polish. Wool bonnet for buffing, fine wool or foam pad for polish.
Acid bath to remove lake scum? Save your money and buy some cheap toilet bowl cleaner. Safe on the gel and all the scum/stains will wipe or spray right off. If you do it over concrete, try to keep the cleaner off teh concrete and keep spraying down/diluting what gets onthe conc. It will etch the concrete.
Interior vinyl cleaning, if it's real dirty, get some Malco Plastic and Leather cleaner, works the best of anything I've tried. Apparently Totally Awesome cleaner form the dollar store is the same stuff, but only a dollar! I'd try that first if I didn't already have Malco. For spot stains on the vinyl, alot of times a blob of liquid wax like Meguires will work. Pour a blob on the stain and let it sit until it dries. It's lifted several stains off of my white vinyl.
I hate hand waxing, sucks even more with a black truck and black boat! I've started using Turtle Wax Ice spray wax. It seems to last pretty well and it's spray on wipe off, no need to go back and wipe off the wax residue and it doesn't turn plastic trim pieces white.
Normal waterspot cleaning vinegar and water with some spray wax mixed in