I second Remco,
in terms of your sponge weathering; its a great place to start, as its a simple but very effective technique. I suggest you start from the bottom and do a little there first. So dab a very dry black sponge (the reason I say very dry is that your likely to not get much paint transfer onto the model this way and for a first time it'll help you control the effect), then again with silver.
By not going above the bottom if you really dont like it you can then simply put your 'mud' effect over the top. For my mud I use powders, I have found there mightily effective, I apply them completely dry (which im not sure everyone does?) and then I seal them straight in with varnish from my airbrush. The reason I do this is pigment fixer tends to turn them into a liquid and I dislike losing the texture they have when done dry and thick. But thats purely my taste (especially with orks).
Tbh Remco really did cover most of it. Although for the oil runs and so forth, have you tried oil washes? recent discovery for me and I find them fantastic to work with, as the oil can take hours to dry which means you can fiddle and remove, touch up, changefor ages before you decide you like it. Plus a little white spirit and they go from being tacky/dry to completely workable and wet again. I'd look up some techniques on it if you fancy it
Although, I dont know if they can be done without, but I use an airbrush, which is how I seal and varnish my models for both of the techniques i've described above. Not sure how well they'd work without this; as the oil washes are good on a gloss varnish to help them run, and sealing pigments with an airbrush prevents them being picked up if you were trying to do it with a brush.