taken from NONA's mailing list discussion
Having just retired from my company, (custom molded rubber goods), after 45 years, I would offer the following:
Do yourself a favor, after installation of any rubber seal that's going to be exposed to the atmosphere, go to a drug store/pharmacy, whatever, and purchase a bottle of gylcerine. Use a cotton swab and smear the gylcerine over the rubber, then use a clean soft linen cloth to rub into the surface, and remove the excess. Rub it well into the rubber surface, do it twice if you have too.
Glycerine is very viscous and will attract dust if not carefully rubbed into the rubber surface. Don't just slather it on, it will smear windshields etc. but can be easily removed from the glass or paint with any windshield cleaner.
Just don't make alot of work for yourself.
Indeed, rubber that's imported (it really doesn't matter from where), is very inferior to rubber produced in the United States. I've seen imported rubber, particularly when it's under a stress as at the window corners, crack in less than one month. Especially if you live in an area with high ozone content, such as the LA basin. The early OEM seals are terrible as the technology just wasn't there at the time.
If you notice a white chalky material on the surface of your rubber products, particularly when first purchased, it's supposed to be there. It's an antioxidant that's placed in the rubber when it's mixed and is doing just what it's supposed to do, bleed to the surface to protect it from ozone contamination. Easily removed with plain old dishwater soap. When you remove it, put the glycerine on.
Do yourself another favor and DON'T use commercial products, (such as the one that starts with an "A" and ends with "L", Silicone is not the answer to a maidens prayer, in spite of what they claim), on rubber/leather/vinyl surfaces. Most of these products are petroleum based and don't agree with the foregoing materials. In fact, they will do the opposite over a period of time and destroy rubber. If you're working with leather, talk to someone that works with leather and ask what you should be using. Ditto for any other materials.
I've had rubber seals on my cars for years and years with no failure due to ozone cracking/crazing. I do the gylcerine routine at least once a year, it's no different than protecting your paint such as you do when waxing.
taken from VWtype3.org's mailing list..
I know this has been said before, but I decided to try using glycerin on my pop-out glass rubber to see how it worked. For anyone who hasn't tried it, it is truly amazing! That rubber basically needed to be replaced as far as I could tell, but it now seals well and looks pretty good too! I reapplied the glycerin over several days because the rubber just kept soaking it in. Once it was all washed off I can't really tell you what a transformation had taken place....
taken from our mailing list...
Date: Sun, 25 Feb 2001 11:35:35 -0600
From: "Peter B. Tirrell"
Subject: Re: [T34] zymol seal protectant
All - I have been using this product for several years. IMHO, it seems to rejuvenate the seals. It has been a life-saver for the seals of my T412 coupe (NOS and reproduction seals are non existent) and has been great for the seals of my 64 T34. Of course, it will not heal cracks, but does seem to give the seals more body and suppleness. I also have used it on original seals of my 60 T1 cab and new seals of my 71 T1 coupe. Use it sparingly - it goes a long way! Clean the seals first with a very mild and weak solution of detergent. Remove all the dead rubber and solution. Let it dry thoroughly. Apply the Zymol by working it in with a foam-tipped "Q-tip" (available from some supply catalogs such as Grio's, but you can make your own). On the 412 seals, which were very dry, I left extra Zymol on the surface and let it soak in for 2-3 days before wiping it off. If you get a lot of beading on the seal, then you probably have wax or something on the seal that will prevent the seal from soaking up the Zymol (needs cleaning). BTW, I wear rubber gloves when I apply it. It is difficult to get off your fingers. I avoid getting it on the cars 'cuz it smears and also is difficult to clean off. I throw away the cloths I use for that purpose.