As far as I know the Testor's spray is juts an acrylic varnish. Future is an acrylic based wax, and will dissolve (on home made decals) when placed in water - I learned from experience !
In 'the old days', I used to brush on ordinary enamel varnish when making decals by hand, but how this would work if airbrushed onto ink-jet printed decals, given the thinners content, I'm not sure - there's a high chance it could effect the printed image.
I've used many brands of acrylic aerosol varnish without problems so far, except once, when I sprayed two, heavy coats, too quickly, resulting in the image distorting and dissolving at the edges.
Two, maybe three, light coats, with a pause for each coat to dry, is the way to do it.