If I want a matt, or semi matt finish (incorrectly known, sometimes, as a 'flat' finish), I add Tamiya Flat Base to the Future, mixing in a proportion, after experimenting, to achieve the desired finish. This is only for the final finishing coat, to give the model the correct gloss, sheen or full matt (flat) finish required to replicate the original aircraft. For decals, a full, high gloss finish is applied, using either a good gloss varnish, or neat 'Future'
(Note that the term 'Flat', when related to paint and painting, is very often incorrectly used to denote 'Matt'. 'Flat' is in fact a process, as in 'flat' or 'to flat', when an existing paint finish is rubbed down with wet and dry or sand paper, to make the surface flat, and provide a 'key' for a following coat of paint, particularly for gloss paint. The correct term for a paint, or finish, which does not exhibit a gloss or other sheen, is 'Matt', with a finish between full gloss and less shine than semi-gloss, being 'semi-matt, leading to a full, dull finish being 'matt') .