I know I have a couple wood felloe wheels that need new spokes. They look good but a little loose. I'm in east Texas and it didn't rain for about 3 months. In the last 3 weeks it's rained here over 2 feet. Those 2 wheels are tight like brand new I guess because of humidity. Is there some type of paint that that you can paint the wheels with when they are dry and they absorb it and stay tight, or paint them with something now and seal them? Just an idea.
There is no magic chemical that makes shrunken 90 year old rotted wood like new again.
Linseed oil is a great thing to treat any wood that is inclined to shrink when too dry. Constantine drove halfway across Africa with loose wheels. I had a wheel that had loose spokes and I shimmed it with veneer, and it is still tight two years later and 2,000 miles. But like Royce says, if the wood is rotten, the only way to fix it is replace it.
Constantine dodged a bullet!
Linseed oil is useless on anything that is out in the weather, it evaporates here in Texas on a typical summer day. It is a fine finish for an old piano or a dining room table though. It won't fix a loose wheel.