[quote=BuddyJim]My project this winter has been to refinish my cabin sole. It is the original flooring and was in very good shape for its age. There was little to no delamination and with some gentle sanding I was able to get the old varnish off. ...

The bottoms have never been sealed and I was wondering what to use on the undersides. I have read about people sealing them with West System Epoxy and was wandering if a couple of coats of varnish would be just as good. If I do seal them with West System Epoxy which product would you suggest I use?
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Lucky for you that your boards hadn't delaminated and could be refinished.   Believe me, you don't want to have the delamination and have to replace them (having had to do the latter project last year, I can assure you that it is a major PITA!). 
I can't offer any suggestions on the topside finishing (I fabricated new floors using Lonseal vinyl expoxied to 1/4" marine grade plywood), but in terms of sealing the underside, I think sealing with epoxy would definitely be the way to go.  I put perhaps 5 coats on our new boards (be especially sure to coat the edges, which is the highest risk area for water entry and delamination) of West 105 System epoxy.  I used the 206 slow hardener, just to give myself  plenty of time to roll it out once I'd mixed up a batch.  You'll use a lot of epoxy to do it; I bought a gallon of the resin and a quart of the hardener along with the measuring pumps; the measuring pumps makes mixing up batches for the job go much quicker. 

When I did this job 4 years ago I used 3 coats of the 105 west system epoxy on the bottoms and edges of the floor boards, the job took a little less than a quart including some delamination repairs. I then coated the face with 6 coats of Minwax Polyurethane. Finish has held up extremely well so far.

I did my floor boards 2 years ago.

Strip old finish and lightly sand.

Repaired a couple damaged spots.

One coat of West Systems 105 just to water seal the underside, on the floor piece outside the head. Two coats polyurethane for the bottom of the rest. 

Five coats polyurethane with a light sand between coats, for the floor top.

Looks as good as new!
 

I just used sealer on bottom side with 3 coats of polyurethane about 12 years ago. I had to repeat the process last year for the topside because moisture got under the topside finish through the screw hole. I had some delaminating and used epoxy between the layers and clamped. I also drilled out the existing holes a bit larger and filled with epoxy and re-drilled the holes. I also needed to patch with veneer in a couple of spots. Next time I will need to replace completely but for now it is still a good looking cabin sole.

Buddy Jim, how about setting up an automatic signature so that we know more about your boat and where you're based? It's easy to do. Just use everyone else's signature on this thread as a model.

Thanks.

Jim,
One little hint for the process.  After sanding and prep, put a single coat of poly on the top side before you do the epoxy on the bottom and edges.  That will seal the top side so that slight over-brushing of the epoxy on the edges won't stain the good side a slightly different color.  After the epoxy dries, you can put on as many coats of poly as you want on the top side, and who cares what the underside looks like.  It took me two boats to learn that!!!  Good luck with the project.