This tutorial will go over the process I used to weather this tank in detail. You can use the same techniques for any vehicle really so I hope it is helpful.
Tools you'll need:
-Various brushes (I used a reaper pro #1 and a games workshop fine detail)
-A piece of foam or two (If you have an army transport case, the pieces of foam you pluck out work wonders, as does the blister packing foam)
-A set of forgeworld weathering powders (I bought set 2 as it comes with some nice pigments, however I primarily used aged rust, light rust, and some bone dust)
-A wooden pencil
Charadon Granite, Boltgun Metal, Chainmail, Khemri Brown, Gryphone Sepia wash, and Delvan Mud wash.
-Oil paint thinner
Prime and basecoat your vehicle, you may even want to give a light dusting of a matte varnish so as to protect the model's finish while you move it around to paint different sides. Prime the tracks separately; I used a rough equivalent of scorched brown primer on the treads then drybrushed with tin bitz and then boltgun metal after which i gave a 50/50 mix of water and khemri brown wash.
Define the amount of battle damage the vehicle has sustained. Every vehicle should have some story to it - this may seem like a silly step but it is what defines the model and gives it character.
Begin the weathering! With your #1 brush go and put down the Charadon Granite anywhere you think the vehicle has has excessive wear, for example In the image below you can see where I added a large amount of granite to the sharp edges of the vehicle as well as around the top hatch. These would be the parts that encountered the most wear and tear as people would be climbing up and down the hull and opening the hatch a lot. The granite color represents the vehicle's "primer" wearing through the final coat.
To get the feel of how the paint would naturally chip or rust I highly recommend looking up images of rusted machinery as it helps understanding the erratic nature of rust and wear immensely . Here is a prime example:
At this point you should have gotten most of the large blobs done. Now its time to use your sponge. If the sponge is cubed or squared tear off the tip to create an uneven end. pointed ends I have found don't create an erratic enough patterning. Also it is a good idea to use multiple sponges with smaller to larger sized ends for different sized jobs.
Dab the sponge into the granite color and then dab a lot off onto a paper towel (similar to preparing a dry brush). When it comes down to it LESS IS MORE as you can add more chipping by creating multiple layers. Now begin dabbing on some paint and even rubbing it along your already chipped edges. Sprinkle the chips around randomly or if you have a spot in mind that say took some severe shrapnel damage layer that spot up. Remember be Random!! Lighter layers!!
Go back and touch up SOME of the areas you just painted with boltgun metal and then on even fewer highlight on top of the boltgun metal with the very sharpest points with chainmail. Remember that the weathering has no rhyme or reason! there is nothing uniform about it - and this is the key to making it look believable.
Pull out the pots of devlan mud and sepia. randomly tap the rivets with sepia and/or devlan mud to create a small pool around it. You then streak down with your brush as straight as you can - since the inks are relatively thin if you mess up you can always wipe it away quickly without leaving much mess. Sometimes the mess is a "happy mistake" and leaves you with a quite realistic effect. You don't have to do this to all the rivets but you can depending on how ramshackle you want the model to look. Keep in mind chaos is key, no uniformity.
After you've finished the rivets go back and examine the model looking for anywhere water or moisture would collect. As you can see in the photo above there is an abundance of rust where the top and bottom of the chassis meet. I assumed water would collect in the groove and thus create a rust point. This is where you would line it with mud or sepia or even a combination of both and continue to use the streak method as I described earlier.
Time to get into your weathering powders. Go back and look for where you think rust is its worst again and add some powder to your brush (preferably a larger brush) and kind of dab it into the recesses of where rust would collect. Blow away or rub in excess powder. Open your oil thinner (make sure the room is well ventilated! This daemon fluid is near odorless but will fry brain cells and make you light headed quicker than flies on gak.) dab a brush in and then carefully drop it on the powders. it will make them spread and should seal them to the vehicle. Have fun with it. Powders are a lot different than paints so test them on something first.
Final touches go around and rub the lead of the pencil on some chipped edges (it gives a very realistic touch you can't get from paints) and then seal the deal with your matte varnish.
Hope this helps give a better idea of how to weather vehicles!