Furniture, Wood and Cabinetry Finishing - Sprayed polyurethane finish is dull

I recently sprayed on a PU finish on the top surface of a cabinet with a "HVLP" spray gun and my compressor, using material from a gallon can that I had used previously to apply a brush-on finish. The surface had been stained and coated with several layers of urethane, then sanded very smooth with 320 grit wet/dry sand paper.

It was not a particularly hot or humid day, and I had good ventilation to get the urethane drying quickly. I thinned with mineral spirits to get a 30 second drain from the viscosity test cup as recommended by the spray gun instruction sheet. I then sprayed a single coat and got a nice, smooth, fluid coating on the surface that looked like glass when it was wet, yet when it dried it was significantly duller than the brushed-on finish applied earlier.

This is only my second time spraying PU, so I obviously don't have a lot of experience. Does anyone have suggestions as to what may have gone wrong?

If the poly is rough, it was sprayed too dry. More than likely the problem is you didn't apply enough poly. Between thinning the poly and only spraying 1 coat you probably didn't get enough build up of poly. I'd scuff sand and dust, then apply several coats by spray, let the 1st coat tack up before spraying the next coats.

Don't thin it according to the gun information sheet, get an information sheet from the poly manufacturer. They will know what viscosity your product needs to be at. The gun people have no idea. I once spent a half hour arguing with one of my gun reps about needle sizes. I finally convinced him to give me the needle size I said I needed (a lot bigger then what he thought) and it worked just as we thought. They (gun manufacturers) don't always know the products well.

Also, try to keep any reduction to around 10% by volume unless you are doing a wash coat. right now you probably do not have enough film on the surface. 3 dry mils would be great for your film build. on the product information sheet it should give you a volume solids number. assuming no reduction, multiply this number by your wet mils applied to get an approximate dry mil. i.e.: 25% volume solids X 4 wet mils = 1 dry mil. remember this does not account for material that absorbed into the wood or material sanded off.

The problem is not all cup guns will atomize the paint at the coating manufacture's maximum thinning recomendation. Ideally a different gun and/or compressor would be used but sometimes you have to use what you have. I've never used a viscosity cup although I might have one hanging on a wall somewhere. You never want to over thin the coating. Experience with a particular gun will teach you how much it must be thinned to spray properly. For a novice you can start by thinning it a little and then see how well it sprays [use cardboard, scrap wood,etc] You can always add more thinner but it's kind of hard to take it away. Sometimes increasing the air pressure will help but make sure you don't exceed the gun's psi rating. Higher pressure can result in more overrspray.