Naphtha is a generic term for a bunch of different chemicals. That being said, Ronsonol is generally safe for most plastics, especially when used in the small amounts one would use to remove foam seals. It evaporates very quickly, so it’s not usually exposed to anything long, unless you submerge it into a container or something. Not every plastic is safe to use with Ronsonol (it will eat through acrylics), but most (not all) of the plastics found in cameras will be safe. It will, however, damage the varnish coated paper blades found in some very old shutters, and can weaken rubber, but won’t generally completely dissolve it. The only issue I’ve had with it and cameras was it made the rubber coating on a Nikon N2000 sticky. It was fine after a few hours, but definitely gave me a good scare. It’s okay to use in short exposures with some kinds of rubber, after all it’s how they regrip golf clubs. But submersion probably isn’t a good idea. So it depends on the type of plastic and the duration of exposure.
Isopropyl Alchohol will dissolve PP plastics. It’s safe for most others, but it’s not as good at dissolving many types of oils and adhesives. My eyeglass wipes prove that, as they’re premoistened in Isopropyl and mainly just smear the oils from my skin around. That’s why I have to wash them with soap and water about once a week.
Acetone will eat most plastics and paint. That’s the one to avoid.
Which solvent to use depends on what you’re trying to dissolve and what you’re not trying to dissolve. The problem with cameras is you generally don’t know what you’re trying to not dissolve is made of. So your best bet would be to do a test a small amount in a small, inconspicuous area first. I generally start off with Isopropyl and then move up to Ronsonol if that doesn’t work. For light seals, I generally find Isopropyl too laborious.
To the OP, some cameras need foam light seals, and some don’t. My Pentax K1000 leaked light pretty bad before I replaced the seals on it. I don’t think my Argus C3 ever had foam, and it’s light tight. My David White Stereo Realist didn’t come from the factory with any light seals, yet it leaked light, so I had to install some. If you want a light seal that won’t break down every 15 years or so, use wool felt and wool yarn. It has to be real wool (preferably black) and not the cheaper, synthetic stuff. It’s more difficult to apply, but it will outlast foam many times over. Closed cell foam should also last longer than the open cell foam they typically used in older cameras. But closed cell foam isn’t always the best option, like for mirror bumpers, as it doesn’t compress as easily.
After you’ve replaced a few light seals, you get good at it and it’s no longer the nightmare it was the first time you did it. Soaking the new foam strips in Ronsonol before applying them helps a lot because it gives you several seconds to apply the seal without the adhesive trying to stick to anything, and then evaporates and leaves the adhesive good as new. Isopropyl would have a similar effect, but doesn’t evaporate as quickly and doesn’t make the adhesive as slick. But if your applying it to a type of plastic that doesn’t like Ronsonol, that’s your best bet. I’ve done more cameras than I can count, so it takes me less than 30 minutes to completely clean, fabricate (I make my own light seals), and install new seals on a typical 35mm SLR. I actually enjoy doing it, but I wouldn’t want to do it for a living.
