I do not use Humbrol acrylics but my rule is use the thinners marketed by the paint manufacturer as they will work.
Coverage depends on how much you thin. Also a bearing is the primer used. If you use a compatible colour you will use less paint. A 17mm Vallejo bottle just about covered the u/s of a 1/48 Wellington. But the colour has a great deal to do with coverage. Black you can almost get away with one coat (that is mean always use at least two coats) Yellow found five coats is needed to cover light grey primer.
Smaller pieces airbrushed use a lot more paint as a great deal is lost not hitting the piece. If you mist coat first coat that is also another thing to be added in.
Needle/nozzle size. I use .2 & on occasion for large areas .4.
There is no rule whatsoever for any of the above it is in the end your experience which will lead you to a good understanding of what you can do with your method of working.
As said I have not used Humbrol. Noticed my small stock of
Revell acrylics are much thicker than my prefered paint Vallejo Air which are prepared for airbrushing. All like the difference in motorway, town & country driving. Miles per gallon & miles travelled are so diverse.
Laurie
