Had wondered how the sill channels were going to be cleaned/de-rusted, as they're a bit big for what was available here, but finally had an idea: make a 'bath' using PVC pipe. And so, did the larger 'head' part in a smaller container first, and then put the long part into a bath made of 3" PVC pipe with a cap glued on the bottom. Works like a charm.
The sill channels were extremely grimy and looked very rusted, and nothing appeared to make much difference. An overnight bath in Evapo-Rust, however, did the trick, and the one that is done came out much better than expected. Still some surface rust at either end, and the inner side of the channel will need rust treatment and paint, but overall not too bad.
The four sill panel securing brackets have already been done, and are in the storage area.