I varnished inside bottom of the cockpit area of my Petre Play S&G to help protect it from UV.

   I varnished all of the interior of my Guillemot mostly for practice but varnishing anything that will be exposed to sunlight is the right approach.  Definitley the cockpit area and maybe even the hatches.  

My WD12 has never had the interior varnished. It's 14 years old and still not chalking. But...

1. It's stored indoors out of sunlight.

2. I always use a spray skirt or spray deck.

3. There's always a cockpit cover on when it's travelling.

4. It has never been south of Maryland.

So it has never really had strong sunlight shining on its interior for more than a few minutes at a time. If you won't be in a similar situation you should probably varnish it.

Laszlo