Testor's Acryl line includes flat, satin, and gloss acrylic varnishes. I have had good luck brushing it on, though airbrushing is safer. Future is notoriously slow curing, requiring 48 hrs or more in warm, dry climates, and more in cold or damp ones, especially if not airbrushed. Tamiya makes an acrylic flat base which might be added to Future to create a flat version of Future for topcoats.