Reference:Merge

The union operation just glues objects together, it does not remove the objects' surfaces inside the union. Under most circumstances this does not matter. However if a transparent union is used, those interior surfaces will be visible. The merge operations can be used to avoid this problem. It works just like union but it eliminates the inner surfaces like shown in the figure below.

Merge removes inner surfaces.

The syntax is:

MERGE:
  merge {
    SOLID_OBJECTS...
    [OBJECT_MODIFIERS...]
    }

The component objects must have well defined inside/outside properties. Patch objects are not allowed.

Note: In general merge is slower rendering than union when used with non transparent objects. A small test may be needed to determine what is the optimal solution regarding speed and visual result.