I have found that using Alclad's Klear Kote lacquer does take down the shine some, but using their aqua gloss leaves a nice, shiny surface. It also takes Micro Set and Micro Sol well.
FYI here is some Alclad duraluminum on the wings and airframe aluminum on the fuselage. They are sprayed over a coat of aqua gloss that was applied directly to the plastic without any primer or anything.
It's toned down slightly when Aqua Gloss is applied, but in my opinion it is not nearly as much as when Klear Kote is applied.
Note the area on top behind the canopy is a different color (polished aluminum), as is the area behind the exhaust pipes that have yet to be installed (magnesium).
I would seal all decals, because I've seen decals that haven't been sealed after 20 years and ugh.
The other thing I've noticed with Alclad is that the Klear Kote can eat into the metal coat if you don't let it dry long enough. It's not humidwhere I live, and it's hot, so it dries quickly here, but I still would wait at least a week. I found out the hard way when I built a Buchon in polished aluminum that the decal film became noticeable because I applied decals directly on the finish, and the Klear Kote toned down the shine, leaving all the areas covered by carrier film (that had initially been invisible) very apparent.
With Aqua Gloss, Alclad recommends waiting six hours. On the P-51 above, I sprayed Aqua Gloss, then applied decals, then sealed again with Aqua Gloss.
-BD-