First of all this is at your own risk. What I am going to show you can potentially damage your lens.
There are two focusing issues you might experience- front and back focusing. In my case my Leica 90 APO f2 lens was front focusing in the beginning. That means that the element that pushes the focusing wheel inside your camera is a little too long (check FIG. 1). I thought it will be good to peel the black paint and check it again. It turned out that my issue required microns of polishing and the paint was too tick. The result of peeling the whole paint was that my lens now back focus. This is much trickier issue as now you have to add a tiny layer of metal to the focusing element.
So if your lens is front focusing you have the easiest issue but make sure to get the very fine file (like the one your wife uses for her nails) and make sure you do not overdo it.
The way I added a layer of metal that does not fall is by soldering.
Make sure your surface is not absolutely smooth. If your surface is very smooth the solder will fall once you apply pressure to it with your file. Heat your soldering gun and heat the metal part with it. This will help you spread the solder more easily. Once you add a tiny bit of solder to the focusing element (check FIG.2) you will have to spread it over. The tricky part is that you have to solder on a very small surface and you have to make sure no solder will fall in your mechanics. REMEMBER when it is time to use the file, make sure to turn your lens so the bottom is facing down. This will prevent small particles to fall in your mechanics.
Grind once or twice and check. Do not apply a lot of pressure while grinding. Repeat this until you have the perfect focus. Use a printed sheet like mine to check your focus. You can get mine from here. Overall it took me about 20-30 min to fix mine and now the focus is absolutely perfect.
Hope this will help you fix your focusing issue. This is very helpful when your lens is out of warranty. If you still have warranty send it to Leica department to fix it for you.