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I definitely wouldn't try to paint the underside with a gravity feed gun. Pressure pot or airless if you are shooting the house paint. This also doesn't look like a true HVLP. But then again its $15 bucks. This gun is similar to my gun I used to do the edges/tight fade on the camo on my truck. Used my HVLP to to the truck in green and the field on the black and brown.Pressure pot the only way to go.
It basically removes the paint "resevoir" from the gun, allows for a bigger one which is attached to the gun by two hoses. It makes the position of the gun irrevelant and allows you to paint longer without refilling. HF also sells them.OK what's a pressure pot and how much do they cost?