What should I use to clear coat my game board?

if you shim up the edges where the game board isn't, a piece of plexiglass would be perfect. you could then seal the edges with clear kitchen grade caulk and it should last for decades

I would not coat it with anything.

For the main rectangle board in the middle I would place a piece of plexiglass right on top of it. You can get separate  L shaped plexiglass for the pieces on each corner. 

Or cover the entire table with one big piece even of plexiglass and play on top of it. There is no reason to raise the plexiglass up off the board. You could put foam dots under the sections that are not part of the game. You may not need them.

Cheaper: you could get thick clear vinyl  used to cover over dining tables. Found at Jo-Ann fabrics and Hobby Lobby.  Sold by the yard.  Use some weights in the corners to keep it held in place.  Put it on an empty table with weights (heavy books) and get the curl out of it first.

This should be your first solution.  Nothing will be harmed if it doesn’t work. 

On sale  now at JoAnn’s.  65% off.  You would only need two yards- under $10

https://www.joann.com/20-gauge-vinyl-54in-clear/79...

Putting any kind of sealer on the board, you  won’t be able to be able to do anything if it ruins the board or if it isn’t applied well or doesn’t dry right. 

I would be asking other gamers/ gaming store what they have done.  

My husband and son have been gaming for years. I would start by not allowing any food or drink on the gaming table. Even water. If it is close enough that if it spills it will hit your hame, move it

I  would  use West System  Epoxy.  The  stuff they  sell for   wooden  sailboats.   Get the  extra  clear  hardener.  I have  used it on   kitchen  counters and a  kitchen table.  Works well.

https://www.westsystem.com/

The fact that you have altered it doesn't mean you should alter it some more. I would forget about the shortcuts and build a box for it. Maybe a nice piece of 1/4" plywood - like maple hardwood. Frame it around the edges also with some nice wood just high enough to hold the board inside and plexi over the top.  This could be a nice thing, with good wood, rabbet joints. Maybe make allowance for reaching under the edge to pull it out. Even then it's very simple. But then I am a craftsman.......

Take it to a top photography store and talk to them. They deal in preserving art prints, old photos and such with photographic grade preservation sealers and processes.

Steel flex epoxy resin. Build yourself a temporary waxed cardboard frame all around the articles that you want to steel flex. Get enough steelflex so that you can cover it a quarter of an inch thick. Once you have your board laid out in a safe place where it's level and where you have formed your mold. Mix and pour you're steelflex resin. After it's completely hard you can remove one side of your waxed cardboard frame and make sure that the coating that you have poured is thick enough for your liking. If not for the piece of waxed cardboard back in place and pour another coat of resin. When you're all done. Remove your waxed cardboard sand the edges so that it looks nice and sharp then use. A handheld applicator to finish the sides that you have sanded what's the top coat of steel flex the result will be a beautiful looking game board

2 pieces of plexi, or the equivalent material now a days. Fist sheet same thickness as the game board, with the size of the game board cut out of it's middle. 2nd sheet same size as the first without cutting out the board size.

If you coat it with anything you will destroy the value. 

Use two coats of yacht varnish. That stuff is very durable.