Quite honestly, the metallizer sealer will retain the glossy appearance, but it will dull the aluminum, as this is what it says on the bottle. This causes it to become more gray, or dark in appearance than aluminum. On real planes the paint for the markers was always semi-gloss in appearance, except when they used flat for camoflage. This was to keep it from reflecting light in the sun making it harder for enemies to see, or shoot from above. A lot of these aluminum versions didn't use flat since they were bright anyway. I wouldn't worry about making the decals glossy, just put them on as you normally would - the film should have a semi-gloss appearance as the real thing does. I have a P51-B that I painted with non-buffable aluminum, and applied the decals as usual. This made it look just like the life sized version. I love its' appearance. I just keep forgetting to post a thread about it.
~ Cobra Chris