My vote is that if you can't stand the covering crumbling off, then simply take the door off and clean it. This would also be an excellent time to replace seals if needed too. I have one N90 that I have left the door as is with it's covering sticky and barely coming off and another N90 that I have removed the covering from. These cameras are not my go to cameras (my F5, FA and F4S are in that order now), so outside appearance isn't that big of a player for me for these cameras.
My experience is similar to yours performance wise for the N90......perfect, infocus results. I love using them which reminds me I should probably run a roll of film through them....it's been awhile.
Just wanted to leave some info for anybody else who has the sticky back problem:
- Since the phone support person couldn't tell me for sure, I sent in my N90s to Nikon's NY service center at the beginning of this year (2013) to see if I could get the sticky back exchanged. They would NOT do a free replacement. I received a letter with estimated charges to replace back and clean/adj. the camera: $200.
(- I finally got someone on the phone from Nikon service after trying for about a week, I said I wanted to refuse the repair, but also complained a bit more to the person on the phone. The person on the phone offered to forward the problem to a supervisor and said I should expect to hear back in a couple days. 2-3 weeks went by, and just as I was getting ready to call them one more time, I received a package on my front porch. The unrepaired camera was inside with an invoice saying that the customer (me) refused repair.)
- Following the instructions described in this thread (and elsewhere), I removed the back and spent about 30 mins cleaning it with isopropyl alcohol and cotton balls. Worked perfectly. The sticky rubbery stuff peeled away and bunched up sort of like a thin film of rubber cement. Back is now smooth black, not too slippery. There was a lot of loose residue, so you would definitely want to remove the back rather than clean around the edges with the camera open.
- I'd read conflicting reports on this, but the little white mode symbols on my film back did rub off (you can still read the markings if you look at the reflections off the shiny back in some strong light).
It is a classic sleeper of a cam - titanium shutter with 1/8000th, and some of the best AF and metering Nikon's ever done.
I too have done the alcohol thing. There are some thicker areas still visible, but they're not sticky anymore.
One thing I found sped it all up: use 99% alcohol (many of us have that for cleaning), and cut some strips of plastic food wrap. Dribble the alcohol on, and then lay the plastic over - this keeps the alcohol from evaporating. Let it soak in for 10 minutes or so, rub & repeat. Speeds up the process for sure.