I have a HF knockoff of a Devilbiss conventional paint spray gun, that works fine, but I have wished sometimes that it was a HPLV gun. Looking at the pictures of the more expensive HF HPLV gun, it looks like it's identical to mine, but with a small pressure bleed from just behing the diffuser down to the top of the cup. It looks like tapping a spud into the side of my gun at the same point, probably with a small orfice to limit flow, and running a tube down to the top of the cup would make my gun the same as what HP sells as a HPVL gun. Any thoughts?
The DeVilbiss HVLP has a much larger air nozzle in the tip and a larger paint valve. We have one; works great, but it takes a lot of fooling around with it to get the pattern right. There's a volume regulator at the air inlet, and every time you change the fan setting you have to adjust the inlet reg, too. Uses a LOT of air.
If I understand what you mean, you're wanting to convert a siphon feed cup gun to pressure feed. A pressure feed gun isn't the same as a high volume low pressure gun.
The physical characteristics of a HVLP gun are different than a conventional gun (again, either siphon or pressure feed). A conventional siphon gun with a cup has fairly small air passages in the casting and a small paint needle/nozzle. The siphon is created as the air flows past the end of the fluid nozzle tip.
If a conventional cup gun is used as pressure feed, it's usually because the paint/material is thicker than a common paint mixture. The regulated line air pressure needs to be kept low to prevent bursting the cup, and the cup should be intended/designed to be used for pressure feed. Many flat-bottom cups are not. Another pressure feed method is to use a pressure pot, which uses reduced air pressure to feed paint from the pot to the gun through a separate paint hose. An air hose is still required at the gun to provide the fanned out spray pattern and atomization.
Many good conventional siphon feed guns will spray a nice finish at lower pressures if you use a thinner mixture and open the fluid adjustment more than usual. The spray pattern will be smaller of course. An old practice of spraying detail areas such as door jambs (where you don't want to fill the interior of a car with overspray), was to fold the air hose with your other hand, to reduce the air flow thru the air hose. A better practice was to use a smaller touch-up gun, or an air regulator/valve at the handle of a full-sized gun.
To convert a conventional gun to true HVLP performance probably wouldn't work well because the air passages in the gun will restrict the low pressure air flow.
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