When I did my rear bowtie I painted outside
I taped two pieces of string to my spoiler spread out with painters tape, and tied them to a tree behind the car and then put a towel over the strings to cover from bugs and wind. Almost like a tent. My neighbors thought it was pretty crafty.
I did a rough sketch with paint, sorry its a bit crude.
http://i1283.photobucket.com/albums/...ps7f275e81.png
Just make sure you do two HEAVY coats so if it gets messed up you can peel it off easily.
TAPE TAPE TAPE

best I could think of, good luck to yeh