This was done year's ago when I was still doing work on YUPO in Sandy Maudlin's class.
YUPO is a very white plastic so the paint just sits on the top until it dries, creating some wonderful runs and merges of colors.
At the time I did this, I had a horrible looking bunch of greenery and a pinkish red blossom in the metal jug. It was horrid. I used alcohol to remove it and then a Mr. Clean eraser to get the last bits and worked on it without the flowers. The other glass jars weren't touched. (I had to use alcohol and the eraser because it was spray sealed - something you want to do if you work on YUPO because it can always get moisture in or on it and ruin your finished work.)
It was even framed (at the time I didn't know it was so awful). A few years of sitting in a frame in the garage and the white YUPO is very definitely yellowed where the mat did not cover. Interesting that it changed. Which means the YUPO under the painting is yellowed, too. Not exactly what you want if you intend to keep a piece for a long time?
I won't do anything more with this - it'll just be a piece from the past...I usually tear up and throw away old paintings that I review (once a year) that don't live up to my current standards or that I think can be fixed. I never paint on the back of paintings - for some reason I think, what if I painted something really great and wanted to put it in a show and that bad painting would be on the back giving it a bad vibe? That's just me - I could just play and test things on the back of old paintings, I guess. I just remove the work on YUPO as much as I can and then play with it sometimes when I'm in a rut. I don't use YUPO seriously anymore after hearing about it getting moisture in it (even under glass) and being warped when hung in the sun, etc. Now I know it also yellows after a few years (even when not hung in the sun). I guess I'll stick with quality watercolor paper.