This is a Tutorial for a technique I have developed to replicate the look of heavy wear on blasters or what have you. For this tutorial I will demonstrate using a molded clay part I made for a sculpture class I took.
Materials
-The thing to be painted (sanded and primed)
-Silver or other metallic metal-looking spray paint
-An assortment of water based acrylic paints
-Brushes
-Paper towels (with an abrasive, inconsistent surface)
-Water
-Time
-Skill(can be substituted with Confidence)
I have these things
Begin, taking the thing you want painted after it has been primed and such, I have a few of these.

And painting it again, with the metallic spray paint you chose, silver(color) is a good choice as it can be used to emulate steel, aluminum, nickel and a great many other materials.

I mixed up some light grey acrylic for my primer-emulate and applied it to most of the surface.

Then you wad up a piece of paper towel..
Here's the tricky part; waiting until the paint has dried enough but not too much. If it comes off when you touch it it's too soon. If you wait too long it will be difficult to rub the acrylic off but manageable.

Repeat for other things, red for the handle of this thing.

This shows what happens when you don't wait long enough (on the right), the paint just gets moved around and looks bad.

Then I repeated the process for the third color; greenish, over the primer emulate areas, where the least wear would be recieved.

Now for the grime, first mix up some black-brown sludge looking stuff and apply it to the places with recesses and whatever features you want to exaggerate.

Remove it more lazily than with the other colors.

Looks ok, you could stop here if you wanted to.

There's another step I decided to take; oxidation. For my "steel" I'm using a bright orange. For copper/bronze/brass I would mix a light bluish green.Slather on the areas where the first metallic color is showing, rust doesn't propagate on paint.

More rubbing, pretend it's a magic lamp or something.

It's still a little bright so I decided to do another grime layer.

Then clear-coat the thing: finished product.

And THAT's how you fake decades of wear. Any questions?