I DID IT !
I realize I got you waiting for a long time but I finally managed to anodize the frame of my P5. First of all some comments : although I got some advice to have it surface treated by a professional, I have to admit that I very much doubt you will find anyone here in Europe. There sure are gun smiths who can blacken steel gun parts or do very fine jobs on (expensive) guns for a (very) expensive price. Having it anodized by an industrial company was not an option as they don't deal with private customers and they sure don't want to handle guns. So I realized I was on my own. Fortunately I found very usefull info at 2 sites (
http://www.focuser.com/atm/anodize/anodize.html and
http://astro.neutral.org/anodise.shtml). So I got to work : my father supplied me with the 12 Volt battery and battery charger. The lead, caustic soda and aluminum flat bars were purchased at the local DIY. It took me some weeks though to find a supplier for the sulphuric acid (public sales prohibited in Belgium !). I finally found it at a specialist shop for car parts etc. Told the guy I needed battery acid. Next step was to completely strip the frame and to sand blast it. It took me a 2 hours drive to find a company were I could blast it myself. (I don't hand my frame over to strangers). Back home I masked the exposed surfaces of the steel parts (hexagonal transversal reinforcement pin and the mechanical stop for the decocking lever) with two components polyester resin. Next thing was dipping the frame into a bath of caustic soda (i.e. etching). I almost fainted when - after rinsing in demineralized water - I noticed that the surface got unevenly grey ! After touching up the lighter areas with a soda dipped Q-tip, the result was much better and it even became perfect after washing the frame in hot soapy water. Then the "moment supreme" : dipping the frame into a suphuric acid filled container and putting electric power on it. However I noticed some very small bubbles rising very quickly from the mechanical stop area. I immediately fished up the frame, rinsed it and noticed that the pin was being eaten away due to a very small perforation through the masking layer. Got it removed, reapplied and restarted the process. After 45 minutes I lifted it out, put it in a cook pot (luckily I sent my wife and daughters to the shopping mall, cost me a small fortune though) and boiled the frame for about half an hour. Et voila ! Job done ! Afterwards I got myself a double brandy ! I have some pictures of the setup and the results but as I'm not a computer whizz kid I would kindly ask you if I can PM them to someone who would be so kind to post them on this thread, so all of us can share the experience.
Notice : extreme precautions should be considered when doing this at home. Caustic soda, sulphuric acid and boiling water may cause severe injuries or death. I sent wife and kids away and kept the dog in the garden. People who might try this at home (like myself) will accept full responsability for their own acts. The above described experience is for information purposes only.
(by the way, I noticed that "something changed" to the previous links of my pictures, so the moderator may as well delete them from this thread).