Hey Rambo old chum,
Aaah gotcha - I read your last photo post as saying it's done apart from tracks so apologies matey.
Keep at it. Hit it with another Raw Umber wash - Sienna is too red imo and looks unnatural. Then use a really dark wash as a pin wash to show panel lines and hatch edges etc. Pigments, grime and some dry-brushed silver on them bolt heads on the additional armour to make them pop will do the job nicely - unless you plan to cover it in muck. I didn't intend to make mine look this dusty but I'm damn glad I did as it has to be one of, if not my favourite piece yet! Just keep playing around with things and remember to show some bare metal on the drive sprocket teeth and rust on the muffler/tailpipe.
Sorry if I'm telling you the basics but I honestly don't know how long you've been building armour etc buddy. I'm 32 and have only been building for just over 3 years but on top of 20 years of being a ' WWI/WWII/Vietnam Hobby Historian' which helps me. I discovered the whole modelling thing by mistake!
Keep at it buddy, keep at it. Get a graphite stick too (the most silvery one) - also great for showing bare metal areas and can be rubbed with sand paper to make a pigment. Again - sorry if you know all this!
Take care,
Ben