Hey Wodthing,
That's a yummy looking model and one that any modeller would be proud of - well done
I can see the rust and the dust but, if you think about it, if it's been out long enough to start rusting in small areas I'd bet it would have small chips, footprints, general muck etc.
I know you are asking about muck now anyway so that's good. I'd look at investing in some MIG pigments i.e. rubble dust, various mud colours, pigment fixer and Acrylic Resin to make 'Hard Mud' whick looks great (or you can just just buy a very cheap bag of Plaster of Paris in place of the Resin mix).
AK also make a 'mud mix' pack with everything you need to make good mud - it comes with instructions too.
http://www.creativemodels.co.uk/ak_interactive_mud_set_-p-22816.html
The entire AK range - excellent and I use them all. The 'Streaking' stuff is superb:
http://www.creativemodels.co.uk/paints_pigments_filters_washes-c-5_76.html
The other good option for someone fairly new to Armour Weathering is the Tamiya 'Diorama Texture Paint' which is a big pot full of a sludge-like mix that is very realistic and set's rock hard. You can add pigment and paints to it to vary the colour a bit and just slap it on or use a fine cheap brush to be a bit more precise with it.
Tamiya Dio paints:
http://www.tamiya.com/english/products/871texture_paint/index.htm
They are sold for use mainly as making groundworks, mud, sand etc but boy do they make a great mega muddy model if that's what you are after!
These are a couple of the very first Texture Paints that I bought and below is an old model of mine from my real newbie days - it's not the best model I know but the effect on the wheels is brilliant!
One of my very first models - the Tamiya M26 Pershing with the muddy wheel effect gained from using the Texture paint. It's rock hard now!:
That should get you going. I'd recommend:
- either getting some MIG pigment and making your own 'concoction' with plaster, additional paints poured into the mix, extra pigment and applying with very old cheap brushes - go to a cheap shop and get a selection pack of brushes that go from small to flat, large tipped brushes (you will only use them once so don't use your best Sables)!
- Trying the AK selection or
- trying the Tamiya Dio Paints - make sure you add some colour via pigment and paint so it isn't a single shade and mix well!
Before doing the 'heavy weathering' I'd look at adding some light chips and scratches - the model still has plenty of scope for this type of weathering imo.
As for a Winter scheme? Personally I'd start out from the beginning with a winter scheme in mind. Choose you build and choose how to add your Winter Whitewash i.e. via the Hairspray Method for example or a ready-to-go whitewash for models etc.
I hope this helps - this model at the top that you posted is a prime specimen and will look superb when complete.
All the best,
Ben