I've found from various other projects that Wickes do some decent 2ft x 4ft 6mm thick MDF boards for not too much. Two of these makes a perfect 4ft x 4ft play area. A couple of months ago I made a set of these for my X-Wing games. They store nicely in the garage, are easy to transport and extend the size of our dining table at home nicely.
They also solve a problem at COGS. We have tables like the ones below. Although a good size they have an annoying rim around them which when a cloth is placed on top produces a ridge right through the middle of the table.
Anyway a quick search and I found this video in which a technique for making sea tables is presented. So I thought I'd give it a try. https://www.youtube.com/watch?v=uVrKhu9_Wxo
I had a sampler tin of dark blue paint left over from another project and that was used as the base coat. This dried very fast indeed and I've been able to also apply the other coats of paint. I just used some from my Vallejo collection.
I'm pleased with the way they've come out. I'll leave them overnight to dry and apply some gloss varnish as I have no idea where to get the "Glass Coat" stuff the guy in the video mentions in the UK.