I recommend using a LIQUID DEGLOSSER first, to remove any poly/stain/varnish/etc, then scuff sanding with 220 grit just for adhesion. You won’t get the ugly scratches and you’ll be able to prime with 1 coat then paint 2 coats with the SW Emerald Urethane.
The Emerald Urethane definitely has a place in my practice...I've sprayed roughly 10 gallons since it was first released a few months back. It flows beautifully through my airless sprayer - and I've also finally figured out how to get desired result through my fuji HVLP. (1.8mm Tip 15% dil Ammonia Free Glass)
First time I used it was over All Surface Enamel on a door (w/2 coats Bin). Finish was defect free but client didn't like. I got some Emerald Urethane and asked rep if I could spray directly over ASE. He says no problem.
Sprayed it. No sags. Looked perfect. Then about 300 little pimples presented on each face of door. Graco blamed SW and SW blamed Graco. Rep agreed to test my batch.
Then I started spraying same batch through my Fuji HVLP and got a defect free finish, when applied over a freshly primed substrate. So I figured out the hard way. I find it very forgiving when it comes to Sags. I can get extremely greedy with speed of my passes and it holds well. Its build is thin to modest, at best...and will expose a 150 grit "scratch" on certain substrates....So I absolutely love the rich finish but have to apply 2 coats BIN, apply a heavy coat of Premium W&W which builds depth bigtime, and then I spray two coats of the Urethane. If you spray this over 2 coats of Bin only, the finish appearance is inferior to that of 2 coats of bin plus Premium W&W. Furthermore, I have sprayed some other vendors water based primers that I know are good in place of Premium W&W and it doesn't look as sexy.
What I find extremely curious about SW's positioning of this product (Trim & Exterior) is that it doesn't like water....at all. After 7 days, i tried using both warm tap water as well as Warm Tap Water w/ Blue Dawn to clean some pesky construction dust. It left marks where microfiber towel was...and the marks don't resolve.
So now I've been topcoating my doors and casing lately with a 2 part poly (Ultra Matte) over the Semi Gloss and it looks badass and is bulletproof. The first time I top coated it was 24 hours after my finish coat. Widespread poly cracks everywhere which I assume was out-gassing but damn, 24 hours. So now I wait 72 hours and no issues.