Repair or modification of Boston Whaler boats, their engines, trailers, and gear
If you are mostly trailering then I would try out the bottom as is. If he used a good quality alkyd paint and it is pretty old, then it may be well cured and not prone to softening. Use it a season and see. If it fails then either remove it completely or you can use a barrier coat to seal it and continue with a better product.
Do you mean not painting the exterior at all? He didn't use any type of bottom paint but doesn't remember what kind it was exactly. It has tiny cracks all in the paint but I don't think it's soft or even chipping. I can get a close up picture tomorrow. The main concern I have with not painting at all is the inside will be newly painted, it will have all new stainless steel hardware, and most likely a brand new motor. It seems like it would look funny having an old discolored exterior.
What I meant was if he painted the bottom and you planned on leaving it in the water for any length of time then you could see if it failed or not. If it softened and detached then it would need to be removed or barrier coated. If it did not detach, then just leave it or paint over it. But by all means paint the outer hull especially if it is shabby looking. I was very fortunate that even though the inside was horrible, the outside buffed out and looked great. So if the bottom does not have anti-fouling paint then you can paint right over it with no worries. Brightside polyurethane was mentioned and I use that all the time. It is self leveling so it flows out nicely and looks smooth and glossy. Just pick good drying weather and apply sparingly so it does not run. Interlux offers a good primer for Brightside. If you are equipped for spraying it looks even better. I have even sprayed two cars with it and they looked terrific.