Chris;
I use a primer as well,l for the same reasons Dave mentioned: fills better, sands easier, costs less. I use Interlux PreCoat (which contains microballons), but usually recommended the manufacturers recommended primer just to be safe. That's one area where you can mix systems safely, unless you're using some exotic top coat, which Kirby isn't. I reduce my finish paint with no more than 10% thinner, again as recommended by the manufacturer. In my case, that has always been the 333 thinner in the Interlux one part paints. I believe it's the 121 in the Pettit. Penetrol works as well, as does plain old mineral spirits, found in all of them. Now that I've escaped the corporate world and am free to experiment publicly a little more. I'm going to do so, but will start by following the manufacturers recommendations, modifying from there.