There are more techniques for weathering than you can shake a stick at. And not only do you have to know what to do, but where to do it, and how much of it you should do.
A simple weathering on the M41 could include the following.
A. Rust.
get a bottle of testors "Rust" they come in the tiny rectangular bottles. Paint the entire track in this color. When it dries drybrush the tracks first in gun-metal and then very lightly with a lighter metalic color. Use silver only if you can really controll yourself with the drybrushing
When all is dry, paint the rubber pads on the tracks a lightened black. Black paint mixed with some dark grey about 3:1. Voila! you have a rusty track.
B. Mud
There are many things you can do here. The easiest will be to do the following. You can check my post on the M5 Stuart in Model Rivet board to see the results....
After painting the base coat. Take some artist acrylics, I like to use lamp black mixed with burnt umber, and dilute them in water, so that you have very watery paint (not just dirty water), use this to wash the entire undercarriage. Do one side at a time and sit the tank on the opposite side to prevent the wash from collecting in puddles at the bottom. Make sure your dark paint gets into all the crevaces and nooks and crannies, and rub out any water that collects into "drops". Let this dry for a day. Next take some Polly S British Dark Earth paint and liberally drybrush the area with it. Once that dryes follow up with a drybrushing of Polly S British Middle-Stone.
If you have an airbrush, mix the two colors together and mist it slightly up the fenders and other low portions of the tank.
C. Dust
I have trouble with dust, but I noticed that scrubbing lighter colors of the base coat into the finish, in combination with a dark wash and drybrushing gives me a nice dusty/faded effect. Do the scrubbing after the wash but before the drybrushing
This is certainly not the only way to do it, nor is it the best way to do it. Maybe some of the other guys on here can give you better advice.
Ralph