C.O.
on older wood - I would seal it - but I'm not a professional painter or a carpenter.
When we had our home painted 10 years ago, they power washed it, sanded some more (we are all wood!) and then sealed and put a primer coat on. Do I think it made the wood or the paint last longer? I don't know. The paint still looks good and the wood is still holding up.
Sorry - doesn't sound like I'm helping much, huh? I would research it and find out what other experts say.