Mr2Bill4-
I haven't used Testor's washes, but I have *tested* (kind of a pun!) many types of washes in my search for a good performer...maybe my input will be helpful.
Depending on the type of paint you use for your model, you'll want to keep in mind the type of wash you use. If you're using an oil-based paint, then you want to avoid oil-based washes. If you're using an acrylic base coat, then acrylic washes can be used, only if you're careful and use a light touch, and don't over-work it as you apply it.
However, I haven't found a way to get a decent wash out of any acrylic medium...it seems to me that it always floats over the carrier (usually water or alcohol), and then as the wash dries, the pigment doesn't stay settled into the crevices, which is where you (presumably) want it.
Some people use acrylic paint diluted in water, acrylic thinner, or alcohol...some people use india ink diluted in alcohol. Like I said, I haven't been able to get good results from these. I also tried a commercially available wash from a company, but I didn't like it...it was just a smoky-colored paint that didn't settle, it just toned-down the overall basecoat color...so I gave up on pre-made washes.
My preferred method of wash is to use artist's oil paint (the kind that comes in a tube at the art store), diluted in odorless mineral spirits (turpenoid)...it settles nicely, and stays in the crevices...depending on the base color I'm using, I can quickly mix up a specific blend of colors to make the shadows how I want them...but I also keep a bottle of pre-mixed black/brown that I use for the most common big-area washes over *wooden* ship colors (the kind of kits I build mostly these days)...
The thing to remember with oil-based washes like the one I'm describing is that if it is applied over an oil-based coat of paint, it will soften the underlying paint and can cause the basecoat to come off...no fun.
So I use acrylic basecoats only, and apply oil washes....that way, there's no reaction with the underlying paint...
Some people don't like acrylics for painting...my only problem with it is that it's not as durable fo a finish as oil-based paints, and can be chipped or scraped off easily...but the mixing, cleanup, and lack of fumes make it worthwhile to me....
Anyway, hope that helps...I noticed a tag on your post for the Zvezda Black Swan...is that your current kit?? I LOVED building that one! Please share your progress updates!
Thanks,
Dave
P.S.- Here's a pic of my Santa Maria hull, after just a base coat with a little bit of my oil-based wash...you can see how it stays in the corners and adds some good shading....