I believe the twofold purpose of the sanding is to (1) level out any little humps and bumps and pimples so they don't get exaggerated in the next layer and (2) leave a bit of "tooth" in the now-leveled surface to give the next layer of epoxy (or paint) a better surface to grip.

.....Michael

   Thanks, Michael.  I will make sure the lumps are gone, espeially any left by the wood flour-thickened epoxy.  Boy, I wish someone had told me to be careful about cleaning up while using the epoxy because it will save hours in sanding. Oh yeah, they did...about 20 times. 

Getting close to cutting it...Not sure how I feel about that.

You might find these:

https://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/booth-tools/riffler-rasp-set.html

...helpful in cleaning up the the hardened lumps of thickened epoxy which escaped cleanup around the fillets.  I bought a set after our PMD was done.  They've proven useful for all sorts of smoothing and shaping uses, and every time I use them I wish we'd had them when building Winkle.

The sawing in half is...well...very tense.  Get some reliable help to support things as you go.  If, among your friends and relatives, there is someone who is a better hand as a sawyer than you are yourself, you might offer the saw to him (or her) to perform the surgery.  If you know an orthopedic surgeon with some experience with a bone saw, that might be a good choice.  <;-)

In the end, almost any damage the saw may do to the mating plywood faces can be fixed with thickened epoxy.  We had some areas we had to patch that way, even with my younger son's steady hand working the saw.  It all came out fine in the end.

.....Michael

I just finished my second coat of resin for waterproofing all the parts needed.  I am now doing the final sanding and want to be sure my final finish when done will look as good as can be.  However, as I sand to a dull finish I find the surface is not perfectly smooth as there are still flecks of shine.  If I continue sandiing until a achieve 100% dull finish I feel I am removing all the resin.  Is this normal?  Is the wood still waterproof?  Thanks for any suggestions and help!

What's happening is that you have hills and valleys. You need to sand down the hills and fill in the valleys. If you just sand you will, exactly as you say, remove all the epoxy.

Once you get the majority of the boat dulled, it's time to fill in the valleys. If they are true valleys (large wide depressions), fill them in by applying a thin coat of epoxy just into the valleys and a bit around them. Once cured, sand again, Repeat as necessary,

If they are just pinhole-sized pits, apply a thin coat (or two) over the entire boat and then sand and repeat as necessary.

Have fun,

Laszlo

   Thank you Laszio.  I made it through the waterproofing and have started putting the boat together. And yes, this is fun.

Best,

Mike

   I'm tempted to toss in a couple of ideas on this general topic. As Laszlo indicates, you probably shouldn't sand the epoxy coats too heavily -- just enough to show (and somewhat level) the hills and valleys, but not enough to remove much of your expensive epoxy. (Big hills may need to be levelled with a scraper or rasp!) The surface won't be uniformly scuffed, but it apparently doesn't need "tooth" for the next coat to bond. Perhaps this is because epoxy is slow to cure completely and chemically bonds very well in most cases.

I'm an advocate of sanding the varnish coats even more lightly. In fact, I prefer to apply up to three coats "hot" -- i.e., when the surface is dry to the touch but I can still smell the volatile gases coming off. Depending on the temperature, I can apply two coats a day and the next coat or two on the very next day without any sanding between coats. What I hope to achieve is chemical bonding between the coats, and the smell of the varnish is my clue that it is still probable. 

After three or four coats of varnish, I let it get thoroughly dry. This will take at least five days and it's better to give it even more time. At this point wet sanding allows one to remove the little flecks of dust that settled on the previous coats of varnish. As one wet sands, the smell of varnish fills the air. The smell occurs because the hardest, most oxidized, surface of the varnish is being removed, exposing paint that is still "wet" just below the surface. (You are smelling the volatile chemicals coming off that surface.) Once again, applying fresh varnish to this ever so slightly "wet" surface allows the kind of chemical bonding that will ultimately create a very hard, durable finish.

In my experience varnish takes a very long time to harden fully. I'm talking in terms of months. . . . You can prove this for yourself when you try to deal with any of the annoying drips that you left in your perfect finish. If you slice into those drips with a scraper -- even months or years later -- you will smell the "wet" varnish and may even be able to press a fingernail into it.

My take on all this is, I recognize, fairly unconventional. And I probably have less experience than some. But I built my first kayak (a Folbot) in 1964 and it is still afloat. My Northeaster Dory was varnished with Interlux Schooner Varnish three years ago and is still holding up nicely -- even though "hot" application is not recommended by the manufacturer.