Working in small sections at a time, dab the area with some brown paint. I use enamels since they don't dry as fast as acrylics.
Sprinkle with some dirt...
...and let this sit for a few minutes so it sticks in the paint.
Dump off the excess and this is what you are left with. It is a bit heavy at this point and obscures nearly all the detail.
The paint is still tacky at this point. I take a 1" wide stiff bristled brush on tap the dirt with the end of the brush. Keep the brush perpendicular to the surface. This will push some of the dirt more firmly into the paint and the excess will come off at the same time.
I add more to the sides of the hull, following the same steps. Take a fan brush and dip it in some fresh mineral spirits. The bristles will bunch together making a bunch of "fingers". Take this and gently pull some of the mud down to get streaks like wet mud has slid off.
This procedure is very easy. When I took the photos I did not think to snap any of the brushes I used but you'll get the idea. This works great on tracks too. A word about dirt. What can look better as dirt than real dirt. I tried this after I used the Rust All system. A replacement bottle of their clay that can be purchased seperately is expensive.
If you live in an area where you can get some nice mud... you know the kind I mean... the stuff that felt great squishing through your toes when you were a kid. This stuff holds together really well when a damp handful is squeezed. And remember, I am talking about dirt not soil. This does not need to be good and nutritious for plants. Before you can used it need to be cleaned.
Spread some out on a cookie sheet and bake in a 500 degree oven for about 30-40 minutes. That will kill any critters in there. I sifted mine with a fairly fine strainer and you can see the texture I got in the 2nd and 3rd pic below. If you need it really fine, get a piece of very fine PE screen at the LHS and make a little sifter out of some balsa wood with the screen glue to the bottom.
Now the important safety chat. WARNING! This stuff can produce clouds of dust that will settle on everything...and in your lungs. A respirator is a must. And you need to work kind of gently to keep the dust down.