I couldn't easily get hold of marking blue, but discovered a recipe in
SParey for an acidulated copper suplhate solution which works quite well.
The recipe is really easy - a couple of teaspoons of copper sulphate in a
medicine bottle of water and make acid with a few drops of sulphuric acid.
THe metal has to be oil free or it doesn't work. On clean steel you get a
very durable copper finish, which has a second advantage of keeping out the
corrosion in damp workshops...
17 years ago