Any type of wash over a flat finish will be absorbed by the flat paint darkenening it as Steve mentioned. So for panel line washes it's best to have a glossy finish. You can even brush on Future with a soft wide brush. You will need to apply several coats to build up the gloss, 5 minutes in between. Apply light coats to avoid runs. It is self leveling and will not leave any brush marks.
You have to gloss up the surface prior to decaling to avoid silvering under the clear carrier film which happens most of the time over flat finishes. Just make certain you leave the Future cure 1-2 days to avoid any clouding to the decal areas specially when using decal setting solutions.
Another alternative is to spray Aqua Gloss from Alclad, which is water based also.