Think of it as weakened bleach. If you wouldn't want to put watered down bleach on your decals, don't use. I'd recommend getting a bottle, try it on a very small area to see how it does (use a decal you don't like!). I think it'll be OK as those decals have to endure (and fade) a super sunny, hot environments. I'd be surprised if this product would do any more damage, but hard to be sure.