A primer may be necessary to improve the adhesion of the photoemulsion to the metal. After all, metal isn’t exactly absorbent like paper or even fabric.
Once the photoemulsion has been successfully applied (in a darkroom under red light!) and has dried sufficiently (in the dark), the metal plate with a coating can be placed under an enlarger and exposed there – just as one would do with conventional photographic paper.
Depending on the size of the metal piece, it can then be developed as usual in developing trays – stop bath, fixer and final wash are also part of the process.
Due to the non-absorbent substrate, the washing does not need to be as thorough as that for baryta paper.
Information on photoemulsion can be found here:
http://www.laborpartner.info/de_html/de_blackmagic.htm
Furthermore, there is also the book "Silver Gelatin", which describes how to work with photoemulsion on a wide variety of substrates. The book is also listed in the FOTOIMPEX catalogue.
