Got my Thunder40 ULtra Compact Pro matte the other week. Seems to be a decent gun but the finish just sucks. Dug up the web and it seems to be quite common with bersa 9, 40 & 45
Here's my slide:
here's another, from this blog:
Problem with Bersa Thunder Ultra Carry Pro 45 Matte Finish | Gibberish Is My Native Language
here's another case:
Tinkerer's Blog: Bersa Thunder 9mm HC Pro Review (and comparision with the Ruger P89).
owner sanded down to bare metal to get rid of the blotchy finish: