China Wang Mang Spade - Genuine?

I bought this Wang Mang spade coin in the 1990s from one of the most highly respected dealers of Chinese coins in the US. For a long time I thought the patina looked strange, but only recently looked at it in full detail.

The patina is a more vivid green than typical. It looks possibly painted on in places. You can see what looks like some brush strokes on the reverse. The green stuff scrapes off very easily. It's not like the typical copper oxide patina, which is much tougher. The flakes of the green stuff don't dissolve in weak acid, as copper oxide would. In short, I think it's paint.

What do you think from looking at it? It's 17.39g, 58 x 22 mm.
This is from a dealer who would never knowingly sell a fake. But then why is it painted, or is it?