I'm a die-hard Sears Weatherbeater fan. My family has used it for 30 years on the exterior of an old house and watched it greatly improve over the years. The latest "Best" Weatherbeater is polyurethane, not just acrylic latex as before, and has what they call a lifetime guarantee. It washes up with water. I like the semi-gloss, because it always looks freshly painted. (I am trying to find out now if the newest Weatherbeater comes in semi-gloss. When it first came on the market I could only get it in satin, which is too flat for my taste.
If you have bare wood, I'd recommend at least using a good oil-based primer. My first choice is an expoxy primer (clear) made by Abatron called Prime Coat (1-800-445-1754--they have a Web site). I'd follow that with the Weatherbeater.