Washes

I use artist oils from the tube 99% of the time, and they work really well.

I thin them to around 90% thinners, and apply with a brush over a Futured surface.

After around a day of drying, I remove the excess with a wad of paper towel, folded up and slightly moistened with thinners, wiping in the direction of airflow.
When wiping from a spot where panel lines meet, I go in diagonals, so as not to wipe the wash out of the panel lines.

Always rememebr when using oil based washes, to allow a decent amount of drying time, other wise you will always remove the wash.

QUOTE: I took some thinner, mixed in a little black paint and ran it along the panel lines.

Therein lies your problem ... never run "Along" the panel line, rub ACROSS the panel line. Use a Q-Tip that is barely moistened with the appropriate thinner, not wet, just barely damp, and rub across the panel lines. Rubbing along them will cause the problem you mentioned, it will usually just go away.

Like Zokissma I normally use artist's acrylics or artist's water colors for a wash, and I thin them with a lot of water a couple of drops of dishwashing detergent. Additionally, for best results I have found that the surface must be smooth. A flat surface is asking for problems because you will never get the wash out of the texture of the paint. If you want a flat finish, do your decaling and washing, then spray a coat of acrylic flat over everything.

Here's a pic of my Trumpeter F-105G done that way

A wash is a hard thing to get 'just right' the first few times. First off, I don't suggest using black. It's very rare that you will ever need that dark of a paint for a wash.
Second, thin the paint VERY MUCH. If you find that the end result isn't enough, apply another wash.
Watch what kind of paint you use. Many people swear by artists oils. I have no tried them myself, but they look great. I actually use actists tube acrylics, and they are just perfect.
Have you tried a technique called a 'sludge-wash'?
It basically entails mixing a wash but adding a few drops of dish detergent to slow down the drying time, and allowing you to clean up the mess easier.
Check out Swannys description of this technique:
http://www.swannysmodels.com/Weathering.html

Hope that helps. Good luck, and keep at it.