Welcome to the forum ferni5. Here is how it was explained to me when I asked and it helped alot. I do have to credit Erush as it ihis explaination. This was for an armor build and I don't know what you are building so it could be different.
"Go to Michaels, Hobby Lobby, or somewhere that has artist supplies and get a tube of Burnt Umber and Black Winsor Newton oil paints and some turpentine (my preference) or mineral spirits. Give the whole madel a good coat of future to seal it and get a gloss coat on it and let it dry for at least 24hrs. Then mix the oils with about 1/3 black and the rest burnt umber and mix it up then add the turp or m/spirits. You'll want enough thinner to make it look like real dirty thinner but still be runny. Then just use pointed brush and apply the thinner along raised detail and panel lines, nuts bolts everything. It will run along the edges and you'll see the oils collecting around the detail. Now some will go where you don't want it too but don't worry. Let it sit for at least 2-3 hrs, around 6 is best though then use a q-tip or a small paint brush and get it moist then start wipiing the areas you want to remove the wash. It comes off quite easy and if you don't like it you can remove it all pretty easy too. Go ahead and give it a try, it's very easy and really gives depth to the detail on the model. If you aren't sure, try it on the bottom of the tank first or an old model if you have detail on the bottom. Do a couple bolts or a hull line and see how you like it."
Good luck with it.