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Let the cleats float in the dado.
Do NOT use spar varnish, this table is not a spar.
Do use epoxy. I'd use a slow hardener so you get about 30 minute
pot life at 77F.
Jamestown Distributors and System 3 are good sources for epoxy.
Assemble everything then apply a marine coating.
Epoxy requires UV protection, plan accordingly.
My choice would be Epifanes marine coatings available from
Jamestown Distributors.
SFWIW, Jamestown Distributors is in Rhode Island and I'm in LA
which puts them about 3,000 miles from me so I just drool when I look
at their web site.
They have good engineering data.
As far as button feet are concerned, make your own.
Get some 1/2" PVC sheet.
I had access to a lathe to make mine, but a band saw or even a
disc sander will allow you to make round buttons 1/2" thick from sheet
stock.
C'Bore each button to accept a pan head screw recessed into button.
17/64" dia thru hole in plastic, 3/16" pilot drill in the wood to
attach
button to wood.
Use stainless #14 x 1" coarse thread self tapping pan head screws.
Better figure refinishing every 3-4 years along the coast of the
Carolinas.
More frequently as you go south.
You might get 2 years in Miami Beach.
Have fun.
Lew