Someone mentioned using pledge as a flow improver. I'd give that a thumbs up but it works differently than "Flow Aid" that you'll find at the art supply. Pledge is a lot like several of the "mediums" used with fluid artist acrylics really. I think what it does is help keep the pigment suspended. I'm not a chemist, but I'd guess that Tamiya and Gunze both put stuff in their suspension fluid to do exactly that - would also explain why the pigments don't settle on the bottom quickly and perhaps begin to adhere. Of course you will be giving your finish a "satin" effect with Pledge, but I like that for most uses. (How many things are really "flat?") And if you want to dull the finish there are many ways to do that at the end.
As for acrylic washes, I'll know more soon. AK Interractive is getting together with Vallejo and pushing an "all acrylic" weathering option. They don't claim it's the best, but they do claim you can make a really sweet kit without any enamels, oils and the solvents that go with them. Mig Jimenez has done a DVD with AV Vallejo called "AFV Acrylic Techniques." (Might have to order it from Europe.) Mig Jimenez is a blinking genius and could make a model using color crayons so you have to take that into consideration. However, they are selling acrylic washes designed to be used over acrylic base. (I'd sure toughen the base with a good blast of Pledge: you can get a little off with thinner without wrecking the base that way.) The techniques employed are different however. When using an acrylic wash (several now available from Vallejo) you dampen the surface and work pretty quickly. They have some kind of thick retarder - looks like an artist acrylic medium to me - you can get the same stuff mixed with water that would work almost as well. But you would deploy your washes bits at a time. Streaking and stuff can be done very well with acrylics just don't let it dry. Want quick "rainmarks?" Heavily thin some acrylic Buff with water and lay on the stuff slowly in thin streaks: very impressive effect. That's an old Jimenez trick from his MIG days.
I don't think it would matter what kind of paint you used for a wash really, but I'd sure look for some kind of "retarder." If Tamiya doesn't sell it, Gunze does. Better yet, get some at the art store made by Golden or Liquitex - $6 will buy you and your children a life time supply. Pick up some Flow Aid while you're at it - that will supply your grandchildren too. Mix it with water, (about 10:1 water/flow), dip your brush into it and Tamiya becomes a much better hand brushed paint than it is normally. (Or try the pledge technique someone mentioned above.) Added to a wash, it does slow the drying time which could be a problem with an acrylic wash. Personally, I think acrylics dry brush ok, but maybe pigments would be better for fading. Either that or pre/post shading and using varying hues of the base color with an airbrush. I'm not saying this is the best way, although it would help avoid tidemarks which I think some modelers think are part of weathering - they aren't.
Anyway, I'm finishing a ship that's taken much too long and it needs some pinwashes. I'm going to try the black acrylic wash prepared by Vallejo: pretty sure it will work.BTW: if you don't like solvents, you really want to look into fluid artist acrylics. They're tops on the "benign" scale, but you do have to master a few of their mediums. I've done a few posts on them. They work great. I would recommend, however, using proper artist acrylics like Golden Fluids (a really good brand) instead of crafts acrylics like Apple Barrel or Dreamcoat: the artist pigments are definitely better. (I've heard the cheap stuff work nicely in dios - I'd believe that.) If you want to stay in the model paint world, Vallejo is very mellow, but better yet is Acqua Color from Revell-Germany. I'm collecting Aqua now because the stuff claims to be "self-levelling" and from my use, the claims seems to be very close to the truth. I hand brushed a good hunk of terrain on my battleship and it's almost impossible to see brush strokes. That would be great news for tank camo. And Aqua is water based - zero smell.
Eric