Need some help cleaning residue off of hot tub shell.

[hottub63]

Hey guys, I have a tough situation one that I need some help with. I bought a Hot Spring Envoy a few months back and the previous owner did clean and rotate the filters as often as he should have. The plastic at the top and bottom of the filter started to brake down and it got into the hottub and developed a layer at the surface level of the hot tub. I tried to get it off, but if I get a magic eraser I have to scrub it for about a minute to just get a little bit of it off. It would take forever to clean the entire hot tub like this. It's a very thing (but hard) residue to get off. I can actually scrap it off with my finger nail if I work hard at it. (I've attached a picture of part of my shell so you can see it.)

I was wondering if anyone had any ideas of a product (hopefully something I can pick up a Home Depot or the grocery store) I can use to spray on the shell to help brake this stuff down so that I can clean it off the shell.  I really want to get this stuff off my hot tub. Thanks for your help!

[hottub63]

I actually found the following info online from a hot tub dealer. Any thoughts on which one would work best? Also what sort of brush / sponge do you think would work well on the shell without scratching it? I need something to help me scrub it.

Thanks.

When the hot tub is empty, you
can clean the surface by using a mild, nonabrasive liquid detergent, or specially formulated hot tub cleaner. For general
cleaning, any of the following items can be used: Ivory (soap & water solution), Formula 409, Windex, Spic and Span,
Scrubbing Bubbles, Fantastik, Simple Green, Pine Sol or any product available at your local hot tub dealer. For heavy
cleaning, you may try one of the following cleaning items: Soft Scrub (may slightly abrade surface), Bug & Tar Remover
or OOPS. Do not use an abrasive brush or cleaning agent, as it will scratch the finish. DO NOT USE any of the following items to clean your tub: Acetone, Goof Off or any cleaners that contain esters, ethers, ketones, aromatic or chlorinated hydrocarbons, Isopropanal (concentrations greater than 25%). I

[Jacuzzi Jim]

 I like baking soda works pretty good, my failsafe is brake cleaner.   I use it on neglected Jacuzzi's, I know the shell of a HS is different so not sure of the outcome with brake cleaner, but baking soda should be fine... 

[TwinCitiesHotSpring]

dilute some granular pH decreaser into liquid form...glove up and dip the rag into the solution and use that