I don't think I'd use that on decals. It's really designed for plastic (like on plastic/plexiglass windows in convertible tops or on boats). If it was me (and it isn't, 'cuz I'm not a decal kinda guy), I'd probably use 303 Protectant on them after washing them with standard car wash soap and a microfiber sponge. That should shine them up with a matte finish and give them some UV protection so they don't bleach in the sun. You can order it online or pick it up at most boat/marine supply stores.
Technically, I've been told that you're not *supposed* to use any product on the decals b/c there is no product available that can guarantee that's it's safe for use on decals...so I approached the situation with EXTREME care. I've had really good results and have been really happy with using the NXT generation wax: applying it with a soft, microfiber cloth and being EXTRA careful about not getting it around the edges of the letters where they meet the paint. This has kept them from looking "dry," dull, and have kept those pesky ole water spots away!
Ive been using the maguiars h20 hydro beading detail spray you can most likely find at any pepboys on the flat black decals.....gets off waters spots, bird shyt, smudges, whatever with no problems and hasnt harmed the decals at all
armorall makes stuff that protects and shines plastic, rubber, and vinyl. thats what i use on my decals sometimes. it makes them twice as black, and adds a little shine. it says UV protectant and comes in a hairspray bottle