Baldwin made 622’s boiler bands out of “iron,” according the erecting card. This could mean they were “planished iron,” like the rest of the boiler. I’ve interpreted it as ordinary iron, which would have been painted.
As with #10, 622 sports boiler bands made from adhesive tape, painted with the same Steam Power Black as the rest of the engine. As I was cutting the last of them out, I realized it would have been faster and more accurate to cut them with the Cricut. Indeed, if I ever need brass boiler bands, the thing to do would be to cut gold-coloured vinyl.
Based on the photos below, I think I have the bands straight enough. So I will seal them with a light coat of gloss, and start reassembling the boiler details.