Here is a simple process that can enhance you wash effects and give you a great deal more control. I use weathering sponges....
At the risk of advertising for TAMIYA, which is not what I am intending, these weathering sponges (which come in Medium and Fine) are excellent for controlling the shape and extent of your washes. I have used a TAMIYA but you can also go to a ladies make up retailing shop and get all kinds of sizes there (and the embarrassment to go with it) but they might well be cheaper there.
First off, I am applying a dark wash to simulate oil and stains on the floor of the 17cm Grille, the kind that mixes with dust etc and just slowly spreads. Before applying the wash, use some light pastels or pigments to dust the surface. These would be either earthy colors or grey- you can even use some grimy black pigment very sofly and sparingly (I did).
After the pigments are applied, then carefully add the Pin wash with a fine pointed brush (not the sponge), taking off excess from the (FINE)brush with one wipe on the edge of the bottle rim. Make sure you leave enough though to get a small amount to start feathering out from your brush as you lay a trail that simulates an OIL leak. Once this is in place add some dark spots with an Acrylic based paint (dark glossy spots). Up to this point I have only used a fine paint brush to apply a wash. The sponge is used from here to reduce and (better)shape that wash
Follow the spreading trail with the sponge brush and shape the trail how you want it. The wash lifts into the sponge and disappears, so just dab along and reduce by the amount you want shaping as you go...... try to think of a mechanics workshop floor and the stains you see there - keep it wiggly .
Afterwards you can add more dark oil spots if you wish or some dark brown ones using Burnt Umber +black (oils)- it doesnt have to be dry yet and actually works better while the enamel wash is still moist, as it blends a bit.
This same technique can be used on any other washes and works extremely well on 'line washes' that run along and extend out from a wall or recessed panel edge. These run along the base of the wall/ raised edge and also extend out to simulate built up dirty water and gunk on a tank or in the back of an open APC. The sponge can be applied from the outside in, going towards the wall, making sure that the puddle gets larger then smaller again in a random fashion AS YOU MOVE ALONG THE EDGE. The sponge will control this with exceptional accuracy- sorry I don't have any photos for this procedure .
The sponge can also be used to apply the wash as well.......I will add some more photos to show some other techniques better.
I hope this has been informative and given you some ideas on an effect that you have not tried before or not been successful with. If you would like some more information on this or pictures, let me know and I will add to the post.