Yeah, the seal coat helps.  I use Future, and I use washes of water-based acrylics, and of oils and mineral spirits.  I've never had any issue with the wash attacking the seal coat.

I also use pastel chalks, a cheap pack of chalk sticks I got at an art supply shop. I don't usually buy a product when I can make it myself.

I brush those on over my matte coat, and adjust as necessary, when the varnish knocks the color back a little.  I also mix them with water and dishwashing liquid as others have described, to make a sludge.  I like to use this to depict caked mud on an AFV or a MaK suit.