I had the same problem with my TF GS P-40 that had Matte Finish Monokote; changed the decals out and the new ones were Semi Glossy; I used Top Flite 'Luster Coat-Flat Clear Spray'; I made a Mask to cover the rest of the Wing so I would only get the Flat Spray Paint on the Decal Only, wasn't sure if the any over spray on the wing would show up badly; worked great, no prblems, I'm sure any Dull Coat Spray would work, I happen to have a can of the Luster Coat.