Yes, it is the stain from Mutherties,
I see all the dentures all the time and they look fine. Some look like the first day they were put into the mouth.
You are not doing it right, that's why you have some plaque problems.
The stains or the liquid - surface needs to be put on a sandblasted area. I do it with a 110 microns at 2-3 bars. Then I clean it with water and steamer. At this time, you don't touch the surface with bare hands. I hold it just at one place, were I need to polish it manually.
So you make all the staining effects and then cure with the light, 5min each step. And you can make as much steps as you want.
When the staining is finished and you have the desired effect use surface and put it on the whole surface. The surface is thick and in fact too thick, so you need to whipe it off with kompressed air, so it will be very thin.
Because if the surface is to thick, then in the proces of the light cure, the surface will be micro porouse, because the material is shrinking (smaller).
I then cure it for 5 minutes twice. Why twice, because in this proces we make also heat under the lamps and it is not good for the surface and another thread to be micro porouse.
So I put it in the light to cure for a second time with a pause between cca. 2 min.
When this is final I do a machine polish with the parts that are not threated with surface and then the whole area including teeth. This way, you don't get any plaque.
By just puting surface on and off it goes into the mouth, the result is very bad in fact green
This can be done only for a showing off model or a shelf prothesis
I hope this helps,
Regards,
jan