well your in luck just putting together some tutes for my website and tank dirtying is one of em ..heres a couple of pics and steps more in depth will be done soon....
step 1 after painting tank apply a filter this is made using black ink scorched brown approx 1:1 and about 9 parts clean water washed over the whole model to mask the colours.
step 2...add rust in deepest crevices by thinning Dark Flesh with about 5-6 parts water and running into the deepest cracks and along sills etc.
step 3 add chips ...basic idea is to mark out in black first where you want them add slices scratches bullet dings etc...concentrate on edges but ultimately just use your gut and what loks good to you...Paint inside the black with Dark Flesh then paint on the bottom edge a thin light of a light version of the base colour...
now onto mud....
step 1 go around the bottom of the model don\'t worry about being too neat mud doesn\'t but avoid big crayon like strokes...the paint is black and scorched brown 1:1 and about 4 parts clean water...Paint around all the bottom edges.
add bestial brown to this and paint inside the darker colour starting at the edge and work up do this a couple of times adding fortress grey to lighten it in places....
now paint some chainmail in the odd area for extreme wear down to metal...usually towards the front end as this takes the most punishment....
now it\'s time for a bit of mess...mix up Brown ink Black and Scorched brown 2:1:1 and 5 parts clean water...Take a largish brush size 2 or 3 ..Load the brush with the paint ..Not too heavy brush should be fully loaded but not dripping... Hold at the area you want to create splatter and blow sharply onto it creating a spray...This is great fun and you can easily get carried away but as with all effects just paint what you think looks good for you.....the end thing with painting mud is like in nature the more haphazard and natural an effect you get the better it looks....
The finished battle hardened tank....
This is just one way this is quick and easy I did the damage on this one and 6 others the same in around 4 hours part of a commission....
any other help you need just p.m me I am churning out tut\'s left right and centre ready to do my website.....