I guess it depends upon your printer and decal film.
I used Testor's decals, and a regular, old, spray gloss coat.
It is important to make sure the ink dries thoroughly, before sealing, so I recommend waiting at least 24 hours for the ink to dry (used inkjet printer for mine). When spraying, make sure to apply several light coats, instead of one heavy coat.
The light coat will actually seal, and fix the ink, whereas the solvents in the spray will make it run if you apply it too heavily. Same goes for sealing technical drawing pen ink as well.
2 – 3 light coats should be fine, and then follow the instructions on the can for when you can apply them. Let dry 24 hours, or more.
Then, apply a matte finish spray over that, as above.
Also, when cutting out the decals, make sure not to cut into the inked portion of the paper, but to cut around it instead, just to be sure the ink of the decals doesn't bleed. Presumably, that won't happen if you've let enough time go by for the sealing process above.
When letting the decals rest after printing and before/after spraying, make sure to keep out of the household dust as well that is in the air. I stored my decals in a wooden cabinet while performing the above steps.
