I'll try to answer what I can.
Remember there are different kinds of paints and thinners, and all behave a bit differently. I use Tamiya Acrylics pretty much exclusively. Tamiya thinner doesn't "eat" the paint at all. Windex, which contains amonia, is used to clean the airbrush, paint brushes etc. It's the amonia which reacts with the acrylic and disolves it very well, even cured paint.
Many modelers will spray a clear coat over their models before weathering for several reasons. A gloss coat will allow decals to adhere much better than they would over flat paints. It also helps prevent decal silvering.
The clear coat protects the paint from the next stage, which might include oil washes. I use turpenoid mixed with artist oils for my washes. The oil colors I use will vary depending on what I'm trying to accomplish. An oil wash on an engine will be mostly black with a bit of burnt umber. (I use about a BB sized blob of oil mixed with about a teaspoon or two of turpenoid). When doing armor exteriors I don't use black at all, but use burnt umber, raw sienna or mixtures of the two.
If you use enamels and want to do an oil wash using turpenoid and artist oils, you'll want a gloss clear coat of say Future floor polish (clear acrylic) to protect the enamels from the turpenoid, which will eat away at the enamel paint.
The gloss coats help prevent the oils from the washes from sticking all over the place and helps with the flow of the liquid. This allows more pigments to be drawn to surface details and cracks etc. If you're trying to get the tank to look dirty and grungy all over, then you'd probably do your oil wash after you follow up with a flat coat first.
Once the oil washes have dried, I then spray the entire model in a flat coat to seal everything in and give me the flat look that I desire. On aircraft this would be when I spray the exhaust stains, after the flat coat. For my flat coats, I use Tamiya's Flat Base mixed with Future. The flatest ratio you'd want to go with is about 4 parts Future to 1 part flat base. Any more flat base and it begins to look chalky. By using higher ratios of Future to flat base you can vary the flatness of the paint. Often on aircraft, especially if I'm going for a newer look, I'll use a 5 to 1 or 6 to 1 ratio.
I hope that helps. ![My 2 cents [2c]](/emoticons/icon_smile_2cents.gif)