Hi Pix,
Actually that is a very good question. I've been using traditional finishes for something like 30+ years, both in the Antique Restoration and even Automotive fields. Surprisingly, the new CFC Free waterborne products are very comfortable over the top of most finishes and if adhesion is questionable, a sanding sealer (clear) which also acts as an adhesion promoter is available to precoat. We are talking about VERRRY thin coats of product.... to give you an idea, 10-12 coats when dry/cured is like 1 mil in film thickness. The Stewmac product I used recently is a nitrocellulose base specifically formulated for wood musical instruments.... not like casting resin poured over a burl tabletop somewhere. I've had good results with similar stuff over acrylics, alkyds, polyesters, polyurethanes, and epoxies. Surface prep is paramount, as you want your spray to flow without "fisheyes" and you have to get the feel of shooting it as you cannot shoot for gloss, it needs a "half wet" coating which will level out over the first half of the drying time. If I was shooting a bike or a car, I'd think I was going too dry and would expect to see an "orangepeel" result. Water borne coatings tend to respond to gravity, and threaten to slide off onto the floor at slightest provocation. So light coats are best.
Personally I use a gravity feed HVLP with 3-5 lbs pressure at the gun through a .014 tip. The really nice thing is the long open time which lets you get in there with a sharp probe and extract any foreign objects without losing the coverage locally, and not having to wear a full blown respirator to filter out the solvents. No fire hazard, and better for the environment. Mind you a line air drier is still a good idea, and dedicated air hose that has never ever seen the dumb end of any shop air tool which might contribute a bit of oil into the mix is a good thing. My paint hoses have a strange fitting that only fit the line drier/regulator on the one end, and the only things that will plug in the other is one of my paint guns..... Tru-flate is for impacts, ratchets, grinders, and brute force tools.... Johnston fittings are for paint!
Thanks for asking and sorry for the lengthy answer. I really enjoy this stuff and tend to get carried away.
Take Care;
Doug