If your paint is flat then put on a gloss coat before the decals if you want to prevent them from silvering. If you are planning to do a wash or panel lining post decals then do another gloss coat to protect the decals from being damaged by the wash. After you are done take it back to flat with a flat coat. I'm not familiar with the Tamiya spray paints just the acrylics in bottles but I would think the Tamiya Gloss Clear would work. Wait a bit and someone else will most likely chime in that's more familiar with the Tamiya spray paints and what works best with them as a clear coat. Future/pledge revive it, was the go to for clear coats but they are no longer manufactured. Hope some of this helps.