This must be really old butyl tape or something waxy like that (still doing it's job though!). It is a major pain to remove, but I found the best way was to use a scrapper to remove what I could and then use paint thinner (not acetone - it does nothing) to remove the last sticky bits.
The frames are aluminum and had quite a bit of corrosion build up were the seals were bedded so I trashed the old, rock hard, seals (which were the cause of all the leaks) and then scraped out as much gunk as I could. Vinegar helped to break down the really bad parts.
I wanted to polish the frames so I did a little googling and found that Cream of Tartar works great as an aluminum polish. After the frames were all clean and destickified, I made a paste with Cream of Tartar and water and brushed it on the frame with
Then just give em a fresh water rinse and voila! They look like new (not quite) again! Way easy and finally a task that didn't involve using toxic chemicals!