I guess you know your gen pic shows the back of a three piece dial. There is no point in making a backplate with a central washer like that imho unless the upper part of the dial you are going to use is the front of a 3 piece dial, in which case it should already come with its own backplate.
If, instead, you just want to glue your 37 mm dial on a flat 39 mm brass backplate, which is what many usually do, you should just rub your.backplate on a piece of rough grit (e.g. 180) sandpaper until the paint is removed and the brass shows up.