Ticica, you are absolutely correct in terms of load capacity of a thin layer of epoxy. Linear rails are not very stiff and the majority of the load will be distributed within a few inches of each truck, but the calculations will still show plenty of strength.

I gues I didn't clearly state my concerns with self leveling epoxy, when used as a datum for low stress joints (ie rails) it is strong enough, but accuracy is still in question. For high stress areas (column mounting area on a traditional mill) or for areas subjected to point loads and abrasion (tables) it may not be ideal.

As far as self leveling epoxy goes, I have never worked with the material so I am only speculating on its properties. I have seen .003 in refrence to large setup plates for race cars and spacecraft. I would love to see the data sheets for meniscus formation in smaller areas however. I am assuming there is an area near the edge where the majority of the uncertanty takes place. If this areas is less than a few inches it can be worked around, but if it is too big, your castings will lave lots of useless material around the edges.

The bottom line is build your machine so the sum of all component tolerances is below your desired part accuracy. A flatness tolerance of .003 does not mean that the machine will not be able to beat that tolerance on many parts, but over the full travel, it will add significant unknowns.