what effect are you trying to do.
Seriously get a manual on armour and figures, and you will learn the basics quick...I just picked up Osprey's FJ and SS modelling guides for under 2$ ea(and the SS is a hardcover edt).got them mainly for a step by step guide to Camo patterns and colors...but the other bits, skin tones and such are just interesting to read and see if there is anything I can pickup and learn from.
Do you want dust, chipped paint, surface rust, missing parts, major damage, neglect, dents/dings, exteme wear, normal use wear..alot can happen from the plant to scrap heap..and a little can go along way. and it's very easy to OVER-weather something.
here's a start
www.missingLynx.com
Go to Constructive Comments and Tips and Tricks forums, and run through the galleries to see what other people are doing...and if you want a photo of something model or real..just ask someone either has the photo or info on whatever it is....unless your Aferg asking about 75mm PaK40s and how they were towed by Horses without a limber IIRC from a few months back...I don't think he got the answer he was fully looking for.