Once the varnish has dried (best to leave it overnight) you can apply the oils. These work well for weathering as once they have dried they can be manipulated to create very realistic effects such as streaked dirt, rust and soot. You can either buy oil paints from an art shop (lamp black and burnt umber are good ones to pick up for general weathering) and thin them with white spirit or companies like AK interactive carry a dedicated range of weathering enamel paints that are the perfect consistency. Apply the brown and black into any crevasses and contours on the tank and then leave it to dry, usually about an hour or so.
Then take a cloth and wipe all over the tank to remove the majority of the oil from the raised sections. If you also take a brush (a drybrush works well) and dip it in a little white spirit you can then drag this down the various panels in order to create the effect of dirt being streaked down them by rain and moisture. Once this is done you can apply another coat of matt varnish which will seal in the oils and stop them being wiped away during handling and stop the tanks looking shiny.
A final effect I wanted to add was some realistic mud on the tracks of the tank. After researching techniques I found one using the airbrush which created a realistic spattering effect. To ease the process I picked up AK interactives 'very muddy' set which includes various mud colours and also plaster and pigment powders that can be mixed in with them to make a thicker paste. However you can easily do the same using some brown paint thinned and mixed with water. Basically load an old brush up with paint and then spray the air from the airbrush over it to spatter the colour onto the tracks and running gear. I think you could also easily do this by flicking the brush to spatter the paint too.