On my Singer 128k treadle irons, I didn't use paint, I used stove and grate blackening paste (the sort people used on cast iron range made of graphite) it works well even on rust. You brush the excess rust with a metallic brush then apply the black paste and buff. It's a bit messy and labour intensive but the finish is great and it protects the irons.
It may not be orthodox but it worked well for me 😊
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Last edited by Seb58; 08-21-2019 at 03:45 AM.