Time to share

[Neale]

Plugging screw holes before epoxy pour is, as you say, a recipe for disaster - meniscus around each plug needs removing at the very least. However, a sellotape patch over each tapped hole works fine, doesn't disturb the surface, and you can see where the holes are to drill a clearance hole in the epoxy when set. I had to have two goes at mine, and I had drilled and tapped the holes by the time of the second pour. I did have other issues, but tapped holes weren't one of them. In my case, the bow in the 1800mm X rails was around 3-4mm, and given 3mm wall thickness, machining or scraping were both out of the question!

[Sasquatch]

Thanks for casting advice, I'll stick to scraping.
I'm sure casting works sometimes, but, I prefer more predictable results.

Meniscus wasn't big problem, IN2 epoxy from easycomposites is setting in very irregular way. It looked like skin was forming in random areas and pulling surface up between meniscus on the walls. It was up and down 0.1mm. To be clear : I mixed it well ->electric drill with spiral paint mixer->5 minute vacuum degas->pour. I didnt go with west epoxy as i figured that 20 minutes pot life won't give me enough time for degas.

[JAZZCNC]

I didnt go with west epoxy as i figured that 20 minutes pot life won't give me enough time for degas.

Thats one reason you went wrong. West's 209 extra slow hardener gives you plenty of time and gives hours to settle.
I've used this method on 20+ machines ranging from small to 10x5 machines without any problems. Both with and without holes drilled. I've got a 6 x 4 Granit table to check against and Flatness is to within 0.01 when done correctly.
The key to holes is to use epoxy putty.

That said let me just say that if you're just using the machine as a general router for woods, plastics, etc then your not going to see 0.1mm error across that length.
It's too easy to get caught in the trap of overthinking or trying to reach accuracy that isn't reachable with DIY tools. Without a reference surface to check against then your just pissing into the wind and guessing to the accuracy.