The second. Never pour it back in. And, just to be environmentally responsible, don't dump it - take it to your local hazardous home waste disposal facility.

In the meantime, it's not the end, it's the beginning of your odyssey  in your new boat. The 17LT is a lot of fun and in just a little bit you'll be doing things with it you'll remember fondly in your old age.

Have fun,

Laszlo

Concur with don't put it back in the can. You don't want to contaminate your virgin varnish.

Two other thoughts, generated by seeing the mile-long line of cars waiting to enter our hazmat disposal on the few days a year it opens:

1. Kitty litter. Seriously. Dump your suplus into a small bucket of kitty litter and stir it in. I use scrap wood for stir sticks. Set it aside and let it harden. Once hardened it's acceptable to dispose of it in your household trash as it's no longer hazardous. That's what our waste collection & recycling folks say. I've disposed of dozens of partial cans of old non-latex paint this way. It's cheap, simple and effective. The rules might be different where you live so check first.

2. Put a lid on it. I recently had the "opportunity" to search for small (8-10 oz) plastic mixing cups on Amazon. In my search I discovered they offered lids for some mixing cups. I haven't tried this, but in principle it seems like putting a lid on your mixing cup is no different than putting a lid on your paint can. I'm sure someone will be along shortly to tell us why this is a dumb idea ...

   Hard to believe that I already have enough time in on building kayaks that I'm about to comment on varnishing!

"Putting a lid on it" may buy you a little short term time with your varnish, but it won't work in the long run.  Whenever you have air in contact with wet varnish it will start to skin over, and that skin will always find its way onto your boat.  Annoying!   Guess how I know!

Along this same line I have found that the StopLoss Bags are fantastic!  When I open a fresh can of varnish the first thing I do is transfer the entire contents into one of these bags.  The bags are flexible, so its really easy to burp any bubbles out.  No air, no skin!  The other really nice feature is you can easily pour out as little varnish as you think you may need, so if you're a few square inches short of having enough to finish a section its no big deal.  Doing that with a can is a pain.

Oh, and enjoy that new kayak!!!  You're not far from getting it wet, and the feeling you'll get from pulling away from shore in a gorgeous boat you build with your own hands is a feeling you'll likely never forget.

Dave

   SWEET.

This is EXACTLY the type of info/wisdom a newbie like me was looking for!  I want to keep my varnish contaminant-free (and my garage is the cleanest it's ever been--I mop before every coat application!).  And thanks for the notes on disposal.  I'm always down to dispose of it in an environmentally safe way! 

OK, off to prep & apply the 2nd coat on the bottom!!  :)

Another approach:

Go ahead and pour it back in. Then, both because a non-virgin can gets skin-over and gel-blobs, plus whatever possible contamination from your pour-in, simply resolve to always pour subsequent batches through a cone paint strainer. I'll usually add a cap-full of thinner to my working cup, to replace what may have evaporated from the stuff I poured in and since that skin/gel is an indicator that some solvent has been lost just due to opening and storing the can.

You can usually tell within a few inches of the bow (everybody starts there, right?) if your varnish isn't flowing enough and blending with prior sections, and add further cap-full(s) as needed. 

   +1 on the Stop-Loss bags.  Only downer is the caps strip or split if you aren't easy screwing them back on.  And eventually you get better at how much to pour out for a given varnish or paint task so you have less leftovers.  I tend to use up small leftovers by coating one of my worktops.  I have a couple pieces of 3/4" plywood I use as work surfaces on top of my extra sawhorses. Just big enough to spread out some work and not too big to pick up and set aside.  I now have a varnished one, one in Brightsides Sea Green, and am working on one in Sapphire Blue.  :-)  Actually if you hit the ply briefly with a couple of leftover sanding disks to smooth them and then coat them with your leftovers, they are smooth enough to not snag cloth, and are easier to clean.  The varnished one is nice enough looking to use as a prep/serving table in the yard when grilling.  I flip it over and use the underside for drilling and cutting stuff that leaves a mark.

   So much knowledge!  Thanks ya'll!

And @Mummichog--I'm doing a small similar thing.  I have my hatches, and sanded them last night so I could have them as a "space" to use off any leftover.  We'll see how this goes!  :)