I did my 23 keel with the POR15 system. I was Ok with it, but not overjoyed. I used an angle grinder and grinding wheel plus stiff wire brush wheel to get as close to bare metal as I could (no sand or soda blasting). I then used the POR solutions to clean and "de-rust" the metal, and then the POR15 grey paint. One issue is that one of the solutions (the wash I think) needed hot water, tough to get in a yard. The paint goes on smoothly and covers well, and is one-part. I got a few blisters after one season, and some more after each season (it is about 5 or 6 years now). Most of it adheres, but there are a number of spots around the size of a 50 cent piece where it has bubbled. I suspect the barrier coat approach, though more tedious, may be a better seal. Maybe my prep was bad, but I followed the instructions as closely as I could.
I did not do the bottoms of the wings and bottom edge of keel - way too complicated lacking any sling lift on our lake to raise the boat, and I don't care if that small surface area is rusty - I can't imagine fresh water will eat away much of the massive amount of iron there.