I've tried several aerosol primers. Tamiya fine surface primer is good but expensive. Your typical off the shelf generic spray primers are way too rough, I think. But I find that Krylon Camouflage urtra-flat paint works awesome. It bonds really well and I've had no issues with either Vallejo or Tamiya paints bonding to it.  I get mine at O'reily auto parts. As far thinning Tamiya for brushing, I've never had to do it. But everyone has their own methods.