If you want to try laying down a gloss coat of gray first, to skip having to future a flat coat of gray, I would strongly advise you to experiment on some scraps first.
I tried that with some 1/700 aircraft, and for some reason doing a gloss coat, instead of a flat coat and then coating with Future prior to decaling, just did not work. For me, at least, the gloss coat did no dry with that dead smooth level surface you need to keep decals from silvering. Since that is not an issue with your tanker, try it - but do all the weather on both test pieces as well, to see which looks "better" to you.