I see a lot of war battled guts in my area and have done some amazing bring back to life scenarios.
When they are all dried out and hard, that is a great time to go over the exterior- clip any stray hairs, if there are any unwinding issues, I'll often wick in a tiny bit of Jet brand ultra thin viscosity CA glue with the tip of a toothpick or paper clip. It has incredible capilary action and I've never had an issue with it afetr everything is tensioned back up. Next, wrap the string with a bit of 220 grit wet / dry sandpaper and work over any high / low spots, continue with 400 and finally 800 grit sandpaper- the idea is to smooth out the surface, not take major amounts of material off. Next, wrap the string with steel wool and generally buff it to a nice polish. That completes the exterior. Then, wipe them down with walnut oil- Spectrum brand is my favorite- the type you find in the baking section of your local fancy yuppie grocery store- make sure you DON'T get the type with the vitamine E added as a preservative; it gets a bit funny over time. I currently have a Red-O-Ray that is about 30+ years old for my D string that was brought back to life this way and sounds fantastic.
A lot of folks will tell you to use mineral oil ( I don't care for petrolium products against the fingerboard), or olive oil. Walnut oil is a hardening oil, which is my preference for french polishing and use on the strings. It dries out and hardens, rather than staying soft and oily on the string.
For the other side of the conversation, I've got a customer who swears by soaking his guts in a ziplock bag of olive oil for a couple of weeks at a time. His strings tend to be the most supple that I've ever experienced, but the also swell up the size of a cigarette, break pretty fast, and stretch like mad....
j.