Mickey, The info on the Stew-Mac and ReRanch sites is applicable to the instrument specific applications the materials they sell are designed for.
That being said some concepts are universal. One being the better you prep the surface you are finishing the easier each subsequent step will be.
Get the wood perfect before you whip out the product and you'll be smiling later.
On to specifics:
Warning: the products discussed are intended for professional use and are known to carry significant potential health risks. Proper care should be taken to insure that they are used in accordance with manufacturers recommended safety guidelines.
PPG K36 is a sand-able primer/sealer. It's is designed to spray over existing finishes, bare metal, etc to provide a consistent chemically stable surface for the topcoats to adhere to. Pro refinishers need this to go on fast, bond perfectly to *anything* clean and sand easily. Builds fast refers to the primers ability to quickly build up to a significantly thick level, a couple millimeters actually. in a minimum of coats. Every minute in the spray booth is money so pros need to cut the coats to a minimum and get the next rig in as soon as reasonable. If you can get adequate thickness in two coats you've got a product you can sell. It also needs to flash off or set up in less than 20 minutes as well. More of the time is money doctrine.
Once your primer has dried it's time to sand. In the case of K36 what I found was that id sanded very easy with little effort and did not clog even 400 grit paper, dry. It just came off in waves under the 9" block and contours were like butter. In my case I end up sanding off 75% of the accumulated primer leaving a smooth as glass surface that is perfectly matched for the basecoat coming next.
This is the last time abrasive less than 1500 grit should ever touch the surface of one of my guitars (not that that ever happens:-). In the case of the guitar I currently have in progress the next step is base coat. Since we're talking PPG in this thread I'm using a Deltron DBCxxxx base coat. I mix it with the appropriate reducer and shoot to cover. What this means is I'm not going for thickness or luster or shine, but simply to get the depth of color that I am personally looking for. This should not take more than 2-3 coats and will be fairly thin and dull. Runs, specs and such at this point need to be addressed with care. Super fine pointed instruments and purpose designed tools are best and you only have minutes to act before the finish sets up.
Once you have achieved a the desired depth and coverage of color it time to switch to the clear gun. High solids clears are the amazing. That finish on the new AMG Benz that you drool over in the parking lot at work? Two, maybe three coats and it will last for 25 years. It takes a while to get used to them, though. When they say "medium wet" coat they mean "lay this sucker on till you think it'll run off in sheets, then spray some more just for good measure" :-) I mean this goes on thick. It will flow out so smooth you may not even need to finish sand. Freakin' amazing stuff. Since I've already tossed out a couple PPG products here's another Concept 2021. Technique is pretty key with clears, you have to practice a bit to get a good feel. Wait till it flashes, maybe 10-20 minutes depending on ambient temperature then blast on another coat. If you think it needs another you're probably wrong but you may be able to get away with it.
Clean your guns, change the filters on your fresh air respirator and pop a cool one, you've earned it. The next day you'll need to decide if you need to hit it with 1500 or just a super fine cut cleaner (Maguire's #2, followed by #9 on a random orbit sander).
Assemble and play.
-jeff