Some of it depends on what the kit is made of - is it metal or resin? If metal then you have real life weathering options like really "scuffing" it up a bit If resin then you need to use paint or some such to "simulate" the scuffing.
In general Weathering is very individual to the person doign it and what they "like" to work with. But the techniques are usually "similar". To make something like a gun look "used" I general with a technique called dry-brushing. Start by identifying the type of material it "should" be made from and finding a paint that matches. Then I look carefully at the item and "imagine" what parts would take "wear" in normal use/abuse Stuff like leading corner edges that would rub on the leather of a holster - for example. then I take a small (old crappy) brush and the paint and dip the brush then wipe off 99% of the paint onto a paper towel , scrap cardboard whatever, then I rub the "dry brush" in the direction of the wear along those edges. What happens is that the brush still has a "little" paint on it and it lightly deposits the paint along the egde giving it a "worn paint" look. In general too little weather is better then too much - its easy to go overboard.
After that is doen its a good idea to "weather the weathering" by taking some pastel powder (non-oil based - learned that the hard way :rolleyes) and rubbing it into the paint - just a bit so that the scuffs look like they've been there a while and have "oxidized" a bit. After that I like to seal the result in a clear acrylic
Good Luck
Jedi Dade
