When I did my H26...
I sanded and used Interlux 2000 (epoxy vapor barrier) first because, as I understand it, bottom paint is not a vapor barrier. So, the only way to
get past the water intrusion into the gelcoat (and blisters) is to apply a vapor barrier. It’s expensive. It adds weight. And I would do it again.
I then used Micron Extra. One note on that - it was suggested to use a different color on the first coat, so you can tell when it's worn down and time to re-paint. Problem is, the (minor) flaking I've had has occured between the two colors. As though there is a bonding issue (I used black fist, then blue).