Over on the Scalefour Society Forum, I have asked about the use of metal black as a preparation for painting, and these are my experiences...
Well, firstly, I started off by giving all of the parts that were going to be treated a good going over with Carr's Acidip to chemically clean the surfaces thoroughly, and to get rid of any traces of flux, dirt and so on. After this, it was into some plain water in my wife's jewellery cleaning ultrasonic tank...
This always does a superb job at making any remaining grime and noxious substances literally fall off the model. You can even see the slightly cloudy water streaming away from the model when you switch the machine on. And she is happy as well if you offer to clean her rings at the same time - although preferably without the application of Acidip!
After this, the parts were popped on some kitchen paper in a box to keep the dust off them and left to dry thoroughly in the sun. Such are the joys of modelling in the summer.
After this, I reached for the Casey's Gunblue, and applied it with the end of a cotton bud. Unfortunately this was the result - distinctly patchy.
I'm not quite sure why. The model wasn't handled at all after cleaning, and was perfectly dry. I can only hypothesize that it was variations in the grades of nickel silver that made the difference.
Either way, I followed it up with a thin coating of etch primer, sprayed on, to ensure that the paint would adhere properly. I'd washed the remains of the gunblue off with plain water so the surface for the primer was clean again.
So quite a disappointing result. I still see the merits of using blackening on the edges of models to ensure that any paint chips won't be so obvious. That makes a lot of sense, and I'll be doing it in limited circumstances. I also suspect that the gunblue will "take" better on brass than on nickel silver, so I haven't given up all hope of this method.
I thought that I'd post the results so that you can all see what the outcome was, and perhaps add your own thoughts or experiences.