You're on the right track, just the wrong stuff. High build primer is meant to fill in the pinholes and scratches left by sanding a surface smooth. Unfilled weave is like mountains and valleys to it.

What you need is fairing compound, either a pre-mixed formulation available from the major epoxy manufacturers or your own mix. A formula for your own mix is epoxy, hardener and phenolic microballoons. The microballoons are dust-sized hollow plastic spheres. They thicken the epoxy so you can apply thick layers without it sagging. When the compound cures it's easy to sand. It's very strong in compression, not as strong in shear as wood flour. Never use it for glue.

You mix the stuff just like epoxy and woodflour, except that you use a lot more of the balloons. For filling the weave I like to use 2 coats. The first a slurry, the second almost but not quite too thick to spread. The first coat I apply with a roller, the second with a squeegee.

Once the weave is filled, just sand your board smooth. Then start applying the high build primer. Using fairing compound instead of epoxy under paint is definitely the way to go. It's cheaper and lighter than filling and fairing with epoxy.

Here's a picture of the bottom on my WD12 being giving the microballoon treatment.

Have fun,

Laszlo

I agree with Lazlo.  High build primer is OK for small scratches and pinholes but not larger areas.  I learned this to my chagrin on a kayak.  The paint does not stick all that well to the primer and the slightest rub on an underwater limb for example leaves a big white spot of revealed primer.