I agree with Kevin - I'd try giving it a gentle polish, but using a soft cloth, such as the type used for polishing spectacles, or a piece of cotton wool. Do this lightly at first, to test the surface.
Future might be safer to use than other types of clear coats, but test it on a concealed area first (under the tail for instance) to see if it reacts. I don't think it will, but some acrylic varnishes, and certainly enamel-based ones will.
Personally, I don't normally apply a clear coat to NMF, unless the original aircraft had this, or I want to change the appearance of an area, such as flaps for instance. This is because the tone, shade and texture will be altered by the diffusion of the clear coat, however thin, and can make the metallic particles more visible, giving a 'speckelled' effect. However, I do coat the decals once applied, and only the decals, to prevent them cracking and peeling, which will happen on a NMF surface, given time.
Given that the metallic finish is relatively smooth, there should be enough 'shine' to allow the decals to draw-down and settle well - very well indeed normally. This in itself can be a slight disadvantage, as the decals, depending on thickness, the smoothness of the decal surface and the amount of 'tack', may want to almost instantly stick and settle.
The reason this sometimes happens is due to my not using a softening or setting solution on the NMF, as this can, and most often will, leave a stain, showing as a slightly darker, greyish matt area on the otherwise bright silver, once dry - and it's virtually impossible to remove. However, I get around this by ensuring the decal itself is still wet, and the area is wet immediately before application, using a soft #3 paint brush, or smaller, to apply water. The decal is then placed, slid into final position, and tamped down with a soft cloth, at the same time removing the water. Once this has settled, after a few seconds, it is then gently pressed again, normally using a soft tissue, before a minimal application of Micro Sol (not Set) directly onto the decal, which is then left to do its 'thing'. Being a 'metal' surface, more or less, the decals should settle and stick like s**t to a blanket!
Once happy that they are firmly settled (about 12 hours to be sure), they can then receive the desired clear coat - gloss, semi-matt or matt.
Hope this helps.