Weathering is a matter of opinion to modelers. There are some that build and paint there kits in factory mint conditions, then their are the guys like myself that believe that weathering is a natural step to creating a realistic look. I like to look at historical pictures of what i model, then try to replicate that look as much as my skills will allow.
Practicing these techniques and learning to master them will lead you down that path of perfection and quality that all modelers aspire to achieve. Before you can weather any model you must apply a clear coat to protect the paint layer and allow the washes to flow evenly. Clear gloss will allow this to happen, but the use of clear flat will create more staining.
How do we create the affect of stains a streaks? Start by building a wash, washes are nothing more than super thin paint, normally oil or enamel. I’ll use one part oil paint of a color i think is appropriate, to five or six parts thinner or mineral spirits. Burnt umber is a great wash color, it really depends on how heavy you want the filter color to stand out. The best part of a wash is if it doesn’t come out like you want it to, you can wipe it off and start over. That is one reason you’ll want to have a good clear coat, as i said before it will protect your base coat.
To start use a small brush and apply the wash to an area that water dirt or rust will naturally run down.
Try to be precise with your color, in the picture above you can see that rust is being applied from top downward, don’t be afraid to mix colors in your staining and streaking. Allow this to dry and come back in with a moistened flat brush to elongate and direct the streaking. Another way to spread out the effect is to wet it back down and use a sponge brush to blend and pull it down.
Using the sponge will pull massive amounts of the color off the streak. Since i was weathering a submarine i wanted a subtle effect, if this was armor i would not use a sponge, but instead a flat brush would be used to pull down the color.