Lifetime airgun geek AND scale model geek here...
A good material to seal the decal is clear acrylic floor wax (formerly "Future" here in the US and "Klear" in the UK I think...now "Pledge" something-or-other I think).
Let the decal dry thoroughly, then wipe with a damp cloth to take off any excess adhesive residue. Brush the wax over the decal, it will shrink and dry very hard with no brush marks. I might use a couple coats. It will dry quite glossy but you can blend that in to the rest of the finish with fine steel wool and a paste wax.
(Proof I can apply decals LOL...)