Metallic paint out of an aersol almost always looks shit!!
P38 is a perfectly good filler. I suspect you were either putting too much on at once (deep areas should be built up in layers) or you used too much hardener (very common) and it was going off as you were trying to work with it.
Did you rub down the primer filler, and then the primer before spraying with the colour?
Its amazing how good a rippley surface can look in primer!
Rubbing paint and filler down is extremely tedious - but its the only way to get a good finish.

Oh and dont worry - its a mini! You'll get loads of practice!! Ever done any welding? lol!