I just wanted to pipe up and say that I am more convinced than ever that there was something off with the C2 paint I had gotten last year. I had C2-Icefog. Although I loved the color on the large "chip" (if a 2.5 foot sheet of paper can be called a chip), I did not like the color on the wall at all. Fortunately, with Pirula's help it is now gone (DKC24). But my disliking their color is probably not C2's fault :-). However, regarding paint quality: when I was applying the paint, the paint spattered like the dickens. I had previously painted my trim and I literally had to stop and wipe the trim and baseboards after rolling every wall I did because I got this fine mist *everywhere*. From what I understand, it should not have been like this. The paint store insisted I was doing something wrong and that there was nothing wrong with the paint. Perhaps so, but neither with Benjamin Moore before nor with Pratt & Lambert (Donald Kaufman) since, did I have that problem. Secondly, I'd gotten the paint in Eggshell since I'd read that C2's paints run quite flat and that their Eggshell would be a fairly flat. Well, this eggshell shone like the dickens. It was easily the shiniest room in the house - kitchen and bathroom included where I had used a satin finish (BM, not C2). I like my paints flat and I did not care for the shine I wound up with. I've heard generally good reviews about paint quality from the few folks who have used C2. I would imagine therefore that my experience (negative) is an aberration, but I've never before had an experience where something was painful both during and after the application.

What brand did you use? Was it new primer? Some flow better than others. You should be able to use either oil or latex primer if you're using latex paint. If you using oil paint, stick to oil primer. Brushmarks are likely due to the quality of brush (cheap) and/or over-working the paint. Zinsser makes many good primers. Lots of folks like Kilz. And of course, many brands of paint recommend that you use their primer for best results (Ace with Ace, Ben with Ben, etc.). I read a cabinet painting tutorial recently which recommended Zinsser Bullseye 123 for its leveling ability, so that'd be my first choice if you're going with latex.

This is actually kinda funny. Water resistant board should be less porous than normal drywall, which would means it should technically be easier to prime. That the color is purple may mean that it will take more coats of white paint to actually cover the dark color, but one coat of primer is enough.

Spot prime one area to judge. My guess would be that you will either need to sand everything to make all the textures match, or you’ll need to cover it all with 1/4” drywall and start over. That would mess with your molding depths. Or you could do a knockdown texture on the whole room. Or a long nap roller for a bad stipple effect. None if it is going to be easy. This is why everyone always cusses Bob. There are a lot of Bobs out there committing internet craft inspired mayhem on houses. :-/