For weathering washes, you might like to try mixing your own, from water-based acrylics and water. I use craft-store paints to make washes-brands like Americana, Folk Art, Apple Barrel. The Michael's chain of arts & crafts stores carry them, as do HobbyLobby, and I think Walmart as well. Such washes definitely will not attack clear coats like Testor's product.
I make my own weathering powder, too, with pastel chalks. I got a 12-stick set of earth tones years ago, for ten bucks. It'll last me a lifetime.