GWR gate notice

I don't normally collect stuff, but I have recently been left a GWR cast iron notice, which I am in the process of cleaning up.

When received it was in dirty white with black letters and the "Great" painted out. This, taken together with the quality of the casting and the marks on the back, suggest that it is probably genuine, at least to the extent that it has spent many years outdoors bolted to the top rail of a gate.

However, on removing the paint I was intrigued to discover that under the BR-era white paint was a single coat of black (lettering picked out in white), with no undercoat or primer. Is this usual for GWR notices?

 

Mine was black bitumen paint with white lead...

 

Thanks, that sounds about right for what I'm scraping off, and would account for the longevity of the finish.