Use an acrylic clear coat .

That Testors is a lacquer and really weak when topping it with other solvent based coating like that Tamiya wash .

-- or an alkyd ( enamel ) clear coat if you need to use rattle can .

You actually don't need to clear coat prior to applying other effects like washes and decals .

Try out the wash on some practice assembly that you have painted with acrylic but have not clear coated to see .