a few general questions...

There are basically two kinds of wash.  The pin wash is intended to nestle down in lines, holes, depressions, etc.  to simulate a buildup of dirt in these areas.  The other type of wash is a general area wash where you want to tint an existing color, for things like dust, chalking, fluid stains, etc.  The pin wash is applied with a very small instrument, a needle, pin, or very, very small brush tip.  Capillary action draws (hopefully) the wash into the panel line or depression.  Area washes are just brushed over the area.  The dot filtering method now coming into vogue is a sort of area wash, but may be easier for some folks.

I make a paste out of MMP Powders with regular alcohol over my enamels  but I guess any weathering powder will work. I brush it on thick and let it dry. the alcohol evaporates quick and then once dry, I can fine tune it with a dry brush by removing and blending it .

I applied the mixture to my 1/25 Panther and removed most of it after it dried leaving a light caking.

Here I added then removed some by simply using a dry brush to blend the powders

Update

I actually missed the first part of your question about pin washes.   I like to do this over a glossy surface so that the darker glaze ( mixture of thinner and oil paint over acrylic barrier, or acrylic wash over enamel) will run and settle on sharp details like panel lines, grilles, corners etc. I then spray clear flat acrylic to seal it. The second type of wash is an overall wash to darken or fade. Another type is dot filtering, which is using artists oils in the shape of small dots, most of the paint is removed with a thinner damp wide brush dragged down to create rain marks and subtle variations in the color.

The key here is the use of a barrier as in using an acrylic wash over enamel or a solvent as in turpentine or paint thinner over acrylic. This is done so the wash will not attack the paint.

My two pictures show using weathering powders wet and dry to dirty up and created caked on mud. 

I use a mixture of pumice, plaster powder, dirt from the garden and paint. You can apply it by brush or use an old toothbrush to splatter it on. You can also paint it on a sponge and ron the wheels back and forth over it to give a realistic look.

You can make your jeep look muddy by an overall wash of a mud color,or use pigments to impart texture to your mud.

also,you might want to address these specific questions in the "Techniques" section of the forum

Hi, RBT413. The term "washing" or "pin wash" means weathering. After you paint your model, and then seal the paint with a clear coat, you can weather the model with a specially-prepared mixture of paints, pastel dust, etc. You can read more about it on the home page of FineScale.com by clicking on the "How To" link, and then selecting the "Articles" link, or the "Tips Database". You'll find a wealth of information there on how to do just about anything concerning building any type of model.

As far as making a Jeep look muddy..... Got me! I never weather my models. Sorry!

Good Luck with your project.

Gary Mason