Filling/sealing knots in reclaimed barn wood

I think youd have a tough time getting the spar to dry at 1/4” deep. Maybe a clear epoxy would be the ticket? Ive used Quikwood with good success and they make tinted varieties.

-- "there aren’t many hand tools as awe-inspiring as the #8 jointer. I mean, it just reeks of cast iron heft and hubris" - Smitty

You’re most likely going to want to use an epoxy to stabilize those knots. If you’re really careful with the bar top coating, you could probably use that as well.

-- "Well, at least we can still use it as firewood... maybe." - Doss

There are a lot of the two-part epoxy varnishes out there. Here is one: http://www.woodcraft.com/Product/2004669/7320/System-Three-Clear-Coat-Kit.aspx

I have some in my shop, and for the life of me, cannot remember the name. All I know is I repaired a thick poly clear-coated ash guitar with it, and after polishing it out, it was like a mirror and you could not tell where the problem had been. Filled wonderfully.

-- Paul, Tennessee, http://www.tsunamiguitars.com

my solution is to cover both sides of the knot and flood it with thin CA glue..
wait for thirty minutes and sand. repeat as necessary..
if you want, you may add sawdust or coffee grounds to make for more or less color. works for me.

-- Bruce. a mind is like a book it is only useful when open.

Spar urethane won’t do what you want here. Epoxy would be best and easiest. CA glue will stabilize a knot but not good to fill a bigger gap.

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

when I run into this situation, I turn to epoxy. http://www.gougeon.com/ and swear by the products from these folks. not a plug – just a point of reference.