What about weathering?

the reason the mineral spirits/oil wash took away your base coat is because mineral spirits thins and removes any kind of oil based paint. if you use acrylic paints to paint your model, then apply mineral spirits directly over it, then it's fine.

if you want a 'preshaded' effect on your panel lines without actually preshading, do the mineral spirits/oil paint wash on semi-gloss acrylic paint, stay as precise to the panel lines as possible, then let dry and wipe off the excess with cotton swab. because the paint is semi-gloss, a hint of the wash is left on the surface, giving you a 'pre-shaded' effect.

Ditto to what the other guys say really. I'm fairly new at models but have been painting figures and miniatures for years. Acrylic washes seem to dry too quick but if you use the Tamiya acrylics or oil paints this works much better. I first seal the base colour using Johnsons Klear (or Future as it's known elsewhere) then I use Tamiya X-19 smoke mixed with Tamiya thinners at a ratio of about 1 part paint to 9 parts thinner. Seems to really work well. Once done I add another coat of Klear to seal it all.

I'll bet you didn't seal the model with future first or some kind of clear laquer before you did your wash.

Less invasive washes can be done with acrylic paints (plus water and a drop or 2 of liquid dish soap) and the tempra paint. you can also disolve pastels in water. My preference however is to use oils and enamels -actually, I use ground pigment and turps, but always seal with future prior to.

I used the Tempera paint method described in the Feb 2005 edition of FSM. It worked pretty well on my CH-53E. I'll post a pic that shows the underside of the fuselage has the Tempra paint applied but not "cleaned up." However the sponson has been cleaned up. It's easy to do and easy to fix if you make a mistake. You need to be sure that there's a good gloss coat before you apply the tempra.

I diluted with mineral spirits and the base color was Testors light ghost gray enamel.

did you dilute the oil paint? What was your base paint(primary model color)