Hi all, following kind advice from Mick Bonwick about oil showing through powder weathering, I had a look at this. I thought my findings might be useful if you have the same problem, or you are going to start weathering a loco with powders. These are not meant to be definitive instructions or a recipe success, just my thoughts about what I did and what I discovered, and what I'd do next time. If you cock it up yourself, using any of this material or anything else, then (in the nicest possible way) don't blame me.
How the problem manifested itself...
I applied powders to the loco, sealing with coats of Humbrol matt acrylic varnish as I went. As per what I'd read about online if using the powders method.
I thought it looked ok, but then overnight (not sure how long the problem took) I ended up with a tidemark. This spread through the weathering, as per pics shown by the red lines.