My registration number decals (original) from the dealer are still firmly attached to my 2017 Spark 2UP top deck....are you trying to put the decals on the HULL???? Nothing wants to stick to the hull.
OK, that's what I figured. My original numbers are still attached to the top deck nose area, although dock rash has "removed" bits and pieces here and there....I need to get new ones soon too.
The Polytec hulls on the Sparks are what's called a "low surface energy" substrate - you'll need a specific low energy vinyl in order for them to bond properly. Conventional signmaking vinyls won't cut the mustard. You'll need something like Oracal's 1050HT, Metamark's M7-H or Convex's Pitbull - most manufacturers make their own versions though.