Discoloration Of Enamel Coating On Portuguese Stamps

I have a Scott #80 stamp - King Luis 10R overprinted PROVISORIO that is a very dark color. Most of the enamel coated stamps I've seen from this series have a shiny white or a clear sheen. But, this one has a very significant discoloration. Has anyone else seen anything similar in their Portugal stamps? The picture of the back and the close ups of 2 corners show it to be the coating that is discolored.

From what I can tell, this is just a very dirty stamp. It could be improved by using a good quality soft eraser, like a Mars Staedler one usually found in art supply stores.

I had nothing similar in my collection, but do concur that "enamel"? coating can be a bit strange when you fist come across it.
I agree with HB.

I thought it was just dirty too. Tried a water bath with some soap and also tried a soft white eraser when it was dry. The pictures I took were after those efforts. In the zoomed photos the perf tips look like the enamel coating is actually discolored while the paper is still white.

Oh well, thanks for checking.

It may be that if the discoloration does occur collectors have treated them as heavily soiled stamps and probably discarded them.

Joe

There are 1001 reasons why a used stamp can show a divergence in standard.
It may have come from a badly toned cover, or chemically treated by
a new collector. Whom knows?

Personally, I scan and keep (clear) images, of anything that I do not understand, in case I see it again, or hear or read about similarities.