Good stuff Geo.
I normally apply the decals direct on to a NMF finish, having ensured it's smooth and polished first, then apply a sealing coat of the desired finish (gloss, semi-gloss or matt) over the decals only, using a paint brush. Of course, that's on enamel paint - not sure what the effect will be on acrylic.
Some clear gloss varnishes can have the effect of emphasising the metallic 'grain' of the paint, where the clear coat acts as a lens, magnifying the metallic particles. Although this is hardly noticeable with Future, compared to 'true' varnishes, it still happens, and can totally alter the appearance of the finish. On the main wings, this will not be a problem, and will, in fact, help to create the look of the painted surface, and a contrast to what should be bare metal on the remainder of the airframe.
As mentioned, if the decals are trimmed as close as possible, and the surface is smooth, 'silvering' of the decals is less likely on a 'metallic' surface, and the clear coat over the entire decal, including any remaining carrier film (but not the entire 'bare metal' surface) should eliminate any effects of silvering.
However, what can sometimes happen with age (after a year or more, depending on atmospheric conditions), on a metallic finish, is slight yellowing of the decalled areas, where the shape of the carrier film surrounding, and 'under', a clear decal such as a stencil, can sometimes be seen as a different tone. A lot of this is, however, just natural weathering and, given the decal(s) are properly sealed, it can be cleaned with a damp cotton bud.