Need to cold blue some small steel parts. Seen some youtube channels use various brands and
types but often its only available as a kit and as usual in the UK its super expensive small sub 250ml
bottle costs more than they pay for 25 litres in other countries.
Selenium Dioxide appears to be the most common, safer than hot caustic treatments. But not as
rust resistant and may wear off when handling. These are machine parts so handled a fair bit
but stored for fairly long periods in an outdoor shed.
Cannot be primer or paint, it cannot add any appreciable dimensions to the parts.
Maybe a long shot but worth asking.