Weathering with Metallics
This is probably the easiest way to show your tank or space marine shoulder pad didn't just come out of the paint booth. There is a couple easy things you can do with nothing more than a pot of silver. Edging and scratches are just two ways to add a lot to your paint jobs.
Edging with metallics is the simplest of the two techniques. Simply take the edges of the painted metal surface and apply a metallic paint. I use mithril silver here. This represents the paint on the edges of the metal getting wore down. The same thing can be done if you have rusty metal. Where there is constant rubbing of the surface rust will be brushed off and will not have time to really dig into the substrate. Examples of where to apply metal edging would be on the front underside of a tank where its running over brush, bricks or xenos. It could aslo be on the top side of the rungs on a ladder going up the side of the tank. Below I have an example of a metal face plate for my death korps of krieg horse. I painted it all sorts of rust and grime to represent the corrosive atmosphere. One the edges of the plates where the rider might handle the horses mask I lined it with mithril silver on the edges. It adds a bit of realism and can make things really pop.
Scratches represent glancing bullets that hit painted metal surfaces or just scratches from any other random occurrence that can happen on a battle field. This is essential the same thing as weathering but you don't necessarily have to add it to the edges for it to look right. To do this technique I find a place that I want to add the scratch and first apply black paint. (generally in a thin line) I thin go in with a metallic paint and add and a line inside of the black making sure I stay within the black and don't touch the color outside. This represents something hitting the vehicle or armor and going through the paint and then the layer of primer and hitting the metal before bouncing off. On the red of my land speeder I did this in several places to give the effect that it hadn't been freshly painted and that it had been in the war zone.