Clarify the copper weight intentions on a drawing or "Read Me" file. Note that IPC mandates of a minimum of 8/10ths of a mil in the barrel for continuity (Most fabricators err towards 1 oz. to meet this). So, half ounce starting outer layers would finish at 1.5 ounce or 2.1 mils. Inner layer copper weights should be expressed as a finished copper weight as in a normal multilayer configuration inner layers do not receive an additional plate up, They are basically "print and etch" (exceptions would be blind or buried layers).
Also remember fabricators will typically associate an "etch compensation" based on the known loss from the etcher based on the starting copper weight. A fairly typical rule of thumb is that for every half ounce of starting copper weight a half mil etch compensation is performed on the artwork prior to etch.
Potential problems occur when the space minus the metal etch compensation Is less the fabricators absolute Minimum space. In such cases the fabricator may ask to start on a lighter copper.