I soaked the valance in the old Purple Stuff degreaser over the last couple days. I used a tall kitchen garbage can and was able to get one third of the panel covered, and then turned it over to get the other third. For the center I had to lay it flat and cover that area with a towel soaked in the Purple Stuff and let it sit overnight on the area. That purple stuff works wonders: it removed all the soot and the flat black spray paint. The heavy white paint and primer surfacer is another story.
I had to use a razor blade and shave the white paint off. In some areas the paint and primer layer flaked away together, in others, just the white paint, and in others, neither came off. So I had to use the mini sander, which promptly burned out so now I have to buy another one...
Here's the cleaned valance:
