I'm about to make some welded square tube picture frames and have been
looking into various blueing products. 44/40 runs about $8.50 for 2 oz.
Oxpho-blue runs about $9.50 for 4 oz. Van's Instant is about $10 for 4 oz.
Hoppes runs about 9$ for 4 oz.

I'm wondering what folks here have found to work best on mild steel. Any
hints or tricks?