I use a very simple method. First I paint the entire tank with a factory looking finish. I dullcoat this to protect it. Ususally a couple of good coats.

For weathering I use arylic paint, thinned with water. Use a couple of drops of liquid dish detergent. This cuts the surface tension of the water so it flows on better without water droplets forming. Now you have to experiment with how thin you want this wash. Too thin is better then not thin enough. You can always apply a second or third wash to get the desired effect you are looking for.

With a wide soft brush you just paint the wash on very liberally. You want lots of wash on in a hurry. I have one of those cheap battery powered tooth brushes that I go over the tank with while the wash is still wet. This helps to eliminate any brush strokes. For faded paint use a buff color or light grey wash. Allow this to dry, before the next application. Keep adding washes until you have the shade you are looking for. After the right shade has been achieved, now mix up a thinned black wash. This will pop out the molded in details. Just like the first wash use a large soft brush to try and wash the tank down as quickly as possible. Let dry, and then do it again if required. Again too little is better then too much. Far easier to do another wash then to remove too much. The nice thing about this method is, it can be redone. If you don't like the results, spray the tank with windex, let it sit for a couple of minutes and then wipe the tank down. The wash will be stripped right down to the dullcoat and you will be looking at a factory finish again.

Once you have the right look, then you just go around and highlight some raised surfaces with a light grey of silver, by dry brushing. Dry brushing is the technique of dabbing a small amout of paint on a stiff brush. Brush out the dabbed paint on a piece of paper until until very little comes off the brush. You then move to the model and brush against the raised surface details, like bolt heads, rivets, etc. Womens make up can also be used for this. There are many shades of metalizied eye liner that work great for this.

By this point you will have a pretty nice weathered tank. Now you move to the super weathering stage. This is where you pick an area and do something local and special. On my Sherman I choose to model the effect of spilled fuel. Over time this would fade and some times strip the paint. You do as much of this super weathering as you want. Just doing a couple of areas will be enough for people to notice, and really set your tank off from others.

In my opinion don't over do the weathering . I can't stress enough the importance of using a thin wash applied multiple times over just applying a heavy wash.