7mm scale station sign

This is another of those posts where I struggle with paint. The terrible trio of non-coverage: cream, red and white. OK so I'm not exactly doing myself any favours here and this is one of those instances, although minor, where if you want to go there, you wouldn't start here. 

The hoped for end result is the running-in boards for the O gauge. Perhaps not surprisingly, I've changed the name. Regardless of that I need to knock up a running-in board. There are no doubt special 7mm scale kits for this, but being a lifelong tightwad I'm looking at a few bits of plastic and some Slaters letters that were lurking at the bottom of the scenic stuff box. So far so good, except that the layout has a GWR/light railway vibe and every GW running-in board is white lettering on a black or dark blue back board. The later is easy enough, but the letters are made from black plastic. Black paint over white plastic is OK. White over black ain't.