That gun will not work well at all with that little compressor,it is only 5@90 PSI and the gun's workin CFM is 12..more than double needed for it to work right..you'll get some uneven spraying,globbing of paint..runs trying to keep up..etc etc.Even keepin it down at a 40 PSI will still not be enough to spray a whole car/truck
I've seen those exact compressors..my neighbor has one and did exactly what you are trying to do with it on a tractor,,that tractor ended coming to my house to get stripped,primed and top coated again because i have all the right equipment/tools to spray 1/2 descent because it looked like it was sprayed with a rattle can because the compressor couldn't keep up.
That little compressor will kick on an off constantly TRYIN to keep up with the air usage an HVLP uses,causing uneven spraying,plus everytime it runs it will create more heat in the tank,causing more moisture and heat in the lines.
Anytime settin up a gun and testing for a pattern..you'll find out it usualy takes ALOT more air pressure than the recommended settings from the manufacturer,those are there for federal laws so they can legaly produce and sell it within the federal limits(they have to be able to run at a 10 @ the cap.NOT the input pressure,that is MUCH higher to get the actual 10 @ the cap),but in reality they spray MUCH better at a higher pressure..so that's another downfall on that little compressor.
Those HF "purple guns" are actualy a proven GREAT gun for medium usage or the home hobbiest for the little price..i actualy had that same one but gave it a buddy learning paint..plus i still have the 9.99 on sale Mini HVLP for doing door jams,it has the side cup on it to adjust in any position and it spray AWESOME for 9.99..lol.
As far as what primer is already on it..do a good 220 sand and see how it holds up,,,but personaly i would get rid of it and spray an Epoxy just for insurance it WILL hold up and seal under the top coat