The second attempt at selective aging appears to be a qualified success with lessons learned and remaining issues to resolve. With the area to be corroded masked off, the remainder of the test piece was given two brushed coats of alkyd metal primer. The tape residue was removed with acetone.
The piece looked like this after a one hour soak and light wire brushing. There was no outward sign of corrosion on the painted surfaces, but it may have started.
This is the piece after a total of seven hours soaking and a light wire brushing. Note the corrosion starting in the upper left corner of the painted area. Also note the very slight corrosion working under the paint along the perimeter of the test area. Looking back at it, I believe we would have seen small pits, had I more aggressively wire brushed the area.
This is the piece after a total of fifteen hours. Note the significant corrosion along some edges. Also note the slightly more pronounced corrosion along the perimeter of the test area.
Heavy wire brushing revealed quite heavy pitting, including on some painted edges.
I see two possible causes of the unwanted corrosion along the edges:
1. The edge with the most corrosion was in contact with the bottom of the container and unknown sediments. It is likely best to suspend the work piece, rather than let it sit on the bottom.
2. The paint along the edges might have been thinner than on the flats, but I doubt there were pinholes. In the future, I will try four coats of primer along the edges.
With a little luck, those two adjustments might solve the unwanted corrosion problem. In retrospect, it is also advisable to keep a closer eye on the progress - checking at least every two hours. A seven to eight hour soak appears to be adequate for a mildly pitted surface. Fifteen hours produced heavy pitting.
I would also scrape or sand back the paint around the perimeter of the area to be aged part way through the process to produce a more feathered edge.
None of this applies to protecting the bore. I THINK a heavily greased and tightly plugged bore would be fine, but would not try it without prior testing.
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