Barrier coats are typically epoxy with additional fillers. The goal is to make the hull waterproof, so water can't infiltrate the gelcoat and eventually cause blisters. On newer boats, this is not an issue but older boats like the O'Days you both own it might be. I don't know the history of O'Days well enough to offer an informed opinion.
Gelcoat is slightly porous, epoxy isn't. West System will work as a barrier coat, I'm not sure if they recommend adding a filler. Interlux Interprotect has a filler that makes for a very hard and water proof finish. It can also be sanded and polished to a very smooth surface.
@daviddp If this was my boat, I get a quart of Interlux Interprotect 2000e and apply several coats. Prepare the surface as recommended by Interlux. They won't tell you this, however, if the new 2000e over laps some of the VC17 it won't really matter. I wouldn't do a whole hull going over the VC 17, but a little bit doesn't seem to matter. The VC 17 dissolves in the solvent used for the IP 2K and floats to the surface. I've done several small repairs and it hasn't been an issue.
Why is barrier needed on the outside of the boat but not the bilge?
Two words: Hydrostatic pressure. The pressure of the water pushing against the boat forces the water into the gelcoat. Some areas of your boat will be under 2 feet of water. If you have that much water in your bilge, you have bigger problems than water getting into the hull laminate. It is always good to have the bilge painted to keep water away from the raw fiberglass, but it is not as critical in the bilge as it is outside.