I just picked up a very nice used lens. The only downside is that there's some residual label gunk on the rubberized focus ring (also on the caps, but I can replace them without too much worry).
I'm wondering if anyone has a safe method of removing this worryingly located stickiness? I'm reluctant to try a product like Goo-gone, because I'm not sure what it'll do to the lens. My plan it to do a spot-test with rubbing alcohol, but please wave me off if that would be a bad idea.
Instead of rubbing alcohol, I would go with a higher-purity form of isopropyl alcohol.
There aren't any strong standards on what "rubbing alcohol" contains. If it contains ethyl alcohol, it has to be denatured in order to avoid being classed as an alcoholic drink; its makers do not want all the regulatory problems that go with that sort of classification. If it contains isopropyl alcohol, it typically has quite a bit of impurity in it, otherwise they'd sell it as "isopropyl alcohol."
These impurities are mostly water, but also some things that can leave behind a white haze on things that has to be separately rubbed off. I dunno about you, but I don't want water and haze on my optical devices.
You can get isopropyl alcohol in purities up beyond 99%. It's reasonably safe to use, it evaporates quickly and cleanly, and it is effective.
(100% pure isopropyl alcohol can only be made in laboratories. Commercial grade alcohols are made by processes that inherently introduce some amount of impurity.)
Goo Gone should also work here. I hesitate to recommend it outright mainly due to how little experience I have with it. Isopropyl has done the job for me well enough for so many years that I rarely resort to Goo Gone. The only reason I even have a bottle is that it was given to me.
The worst things I can say about isopropyl are:
Don't get it in your eyes. This is perhaps surprisingly easy to do when scrubbing something with it, since it can aerosolize. A tiny speck of alcohol in your eyes hurts. Wear protective goggles.
Because it isn't the most aggressive solvent out there, it can sometimes take more elbow grease than other alternatives. As long as you aren't rubbing on painted areas, this shouldn't be a serious problem. Just take several passes, and switch out for clean cloths/cotton swabs from time to time as the previous one gets filled with dissolved goo.
It isn't great for your skin. Avoid contact with your hands if you can, or use gloves if you can't.
Avoid anything containing acetone. That will damage plastics, and can remove some types of paint. That's why it's sold as "nail polish remover." According to its MSDS, Goo Gone does not contain acetone, so it may be fine here. I just don't think it's actually necessary.
