From what I could enquire, paint or repaint jobs are all technically diffrent... and so are the original paints from Leica. The best quality is reached on the BP M4. You just have to see side by side a BP M2 or M3 and to compare it with one of those M4 ! It is striking... it seems that M4 do have double layers compared to the previous... and it is even more obvious compared to other
PB Ms (MP or M8-2 finish are quite thin and won't last consistently)
We did investigate these issues as my younger sister has a decorative arts diploma in lacquer paint work... (1 school only in Europe... dedicated to deco arts restoration) so we were interested to give a closer look at what are all these paint jobs about... and what she has found is that noone really does a proper lacquer work... (the best one seen, so far, is the Kodak Bantam Special of 1936...)
Most do an airbrush job, one or 2 layers, more or less thin... (mostly car paint) and polish ... (or even not). Some others do use enamel type paint and cook it... (which has some sense due to the durability of such finish)... but real "laque" work it is not ! (And we could regret it understanding the specific qualities of real lacquer paint ) But in the end, it is to be understood mostly as being too costly to make any real meaning just for a camera body (not speaking of the craftmanship implied in such kind of job) ... so in the end it is all about compromising, technically, for the best result within a realistic price to be asked for... (Of course, if money is no problem, everything is possible: silver or gold leaves for the engravings, etc...!) ... So, user's finish or collector's restoration is finally a real issue while looking closely at this kind of job!
She has ended up doing my MP that was purchased worn over (even a bit outrageously to be honest)... but I am still awaiting for having it reassembled by an official repairman here in Paris... she technically did what she considered the best mix of the traditional technic and cost effectiveness to reach the best possible finish (smooth-smooth-smooth) and durability.
Being her first camera, she chose to let her creativity go and played with the classic M lines, allowing brass to be intentionally showned (and vernished) to enhance this classic design masterpiece... I am sure it shall not be to the taste of everyone... (cf: "camera is just a tool", etc. or "traditionalists" who considers that it is sacrilege to "touch" the M line etc... again "user's vs collectors"... but that is not the discussion here)
In the end her technic can be summarised as: previous finish removal (how depends on its specificities and if dings or dents needs to be fixed or not), 3 thin layers of lacquer paint, each one being polished by hand, and a final vernish, hand polished too... Of course, many diffrent types of finish can be obtained this way: mat, semi-glossy, glossy... with diffrent depths of black, etc...
I will post pictures here as soon as I get this camera back.