I use MM Acrylic Thinner-and add according to the viscosity of the paint. I always add a few drops as I would hope that the thinner has a retardant in it. I have tried mixing some alcohol w/retardant and using that, but haven't gotten the mixture correct....yet.
As far as primer, if it is an interior part with no masking, no primer. Exposed surface for aircraft, I use Tamiya spray can primer...keeps the paint from lifting up. For an armor project where surface texture isn't as critical, I use Krylon primer.