Im not an expert or anything, and I'm sure someone will step in if i'm wrong.
But, that appears to be a glaze of sorts (used when firing in a kiln and then it becomes hardened and glossy).
You want something more like an automotive clear coat. look into maybe "Killer Cans"(pricey but the best you can prolly get in spray paint)
or just step into the spray paint section at walmart and ask for an automotive clear coat. At least get something designed for plastics.
You might also wanna give This STICKY a read through.
edit: reading > me that can is non-firing, but still the same concept. For a car you want automotive grade or something weather resistant.