I opted for extruded foam for a layout base. then I used caulk to secure the WS foam roadbed OR the “craft foam sheets” from AC Moore for the yard area and used caulk to secure the track to the WS faom or craft foam roadbed.
There are staunch supporters of both WS Foam and for Cork.
There are those who oppose the other as well.
There is no right or wrong answer here.
Cork can dry out and crumble, perhaps that is why you heard of varnishing it first. MAny use sheet cork for yard areas. SOme even suggest {gasp} soaking the cork in water to make it more flexible.
The WS foam some just don’t like and nails are not good for it. SHeet craft foam is about the exact right thing needed for large areas I found.
I found that the WS was each to work with and cut and the craft foam easy to cut as well.
What’s interesting about cork is I have some that was stored in my mom’s attic, which is not heated or cooled, and is uninsulated. It’s probably nearly as old as I am (over 40). Some is dried and crumbly, but some is still nice and flexible. I have some other that is less than 20 years old, nearly all of that is dried and stiff and just snaps if you try to bend it.
In a more controlled environment, plus covered with ballast and glue, the cork should last longer.
After using WS and going back to cork, next time it will be WS, if it’s still around. About the only thing easier with cork was around turnouts, and that’s because I cheated and bought those turnout pads.