To me, that seems like a waste of time, money and varnish. Varnish is to protect the epoxy from UV and to make a wood surface look good. If it's not going to be in the sun or visible, there's no point.   

But you're the builder and it's your boat. If it's important to you, go for it. The varnish merchants of the world will love you.

Laszlo

   Laszlo, you are correct that varnish is to protect the epoxy from the harsh UV rays of the sun so I can see why it would be unneccessary to varnish the inside of the compartments but the cockpit will still get a fair amount of sun shining in. I take it that you do not varnish the inside of the cockpit?

PS

   I live in Florida and on my first kayak I didn't bother varnishing the inside of the cockpit. I really didn't think it would get that much direct exposure to the sun. After about ten years of use I noticed the epoxy was starting to get a bit cloudy. Fortunately it was just the surface and I was able to do a light sanding, rolled on a very thin coat of epoxy and then varnished from the rear cockpit bulkhead to about two feet forward of the coaming. Haven't had any more problem. Now I varnish the cockpit on every kayak I build. I don't do the storage compartments as the hatches aren't off that much in direct sun. I suppose if I used a spray skirt and put the cockpit cover on when I'm out of the boat I wouldn't need to varnish. 
 

George K

 Peter,

Sorry, I misunderstood your question. I thought you were asking about the inside other than the cockpit, like the hatches. While I would never varnish there, I can see varnishing inside the cockpit, depending on your situation.

FWIW, I live about 11 degrees north of George and don't get the near-tropical sun that he does. I also use a splash deck or spray skirt when I'm paddling, so no sun gets into the cockpit. Finally, I transport my kayaks with cockpit covers on and store them between uses in the back of my garage or in the basement, out of the direct sunlight. With all that, my boats get a lot less light than George's, so I can get away without varnishing the inside of my kayaks at all. If I lived in the sunny South, I'd definitely varnish the interior.

Your idea of varnishing before attaching the deck is sound. People do that with paint all the time. You just have to be careful to mask carefully and don't leave the masking tape on too long or it gets to be a pain to remove.

Laszlo

���I live Florida too. I often cover the cockpit with a skirt or cover. Even during lunch stops. UV is bad but spiders and lizzards give me pause. Besides sun on a black seat hurts. I don't varnish insides. It is extra unnecessary weight.

   HOWEVER if you want to build boats like Nick Shade does you will varnish the inside.