A local dealer recommended to coat the whole exterior of the thru hull with a 3M sealant, not sure which, it was more like an epoxy, or to coat them with a thin coat of 5200 to give yourself an extra season and to be safe, but honestly I would not recommend this at all nor feel comfortable with that. I'm not sure how this dealer thinks that would solve a problem if the deterioration is the whole way through the nylon plastic thru hull, it will not fix it. There is NO shortcut or home remedy for this fix. This is something that owner's of older boats need to accept and attend to, at some point they will need replacement and the only way to fix it is the right way.
The only safe way is to replace them. Once they get chaulky looking and cracks appear, your pushing your luck. 4 Season's ago 5 boats in my marina became submerged after a storm, 3 were pre 1990 Grady's with plastic thru hulls. They did not sink, but were sitting super low, the marina caught it and hauled each boat before they could sink...the beauty of having someone always at the marina and personnel living nearby and the marina having a set of keys for each boat. Replace them with Brass over bronze or SS thru hulls. They might be hard to get to, especially on some of the smaller models. Some might need access hatches cut into the boat to access them, cut them to an actual hatch size, there are some 9" and smaller circular ones that work well. I know it is inconvinient, but the only way to be safe and to insure your safety on the water as well as your investment into your boat. Unless you have lots of experience with fiberglass, leave this job to a yard of fiberglass shop with experience, they will access it much easier and save you your knuckles. It will cost more, but be done correctly and quicker without stressing you out too much...except for that missing money in your pocket. Do it the right way, not really worth cutting corners.