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Understanding Transfer Efficiency
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From contributor K: Transfer efficiency is a comparative guideline only. The term refers to the ratio of finish that actually stays on the sprayed item compared to the amount sprayed. The losses come from primarily from overspray. Think of the billows of clouds that are created with a compressor-driven gun - that is finish that doesn't hit the item. Compressor-driven spray guns are typically credited with being in the 30%-35% efficiency range, HVLP's in the 70% range. Some air-assisted airless units purport to be in the 95% range. The percentages are only a relative and test lab indication. If the exact items are sprayed comparing compressor-driver and HVLP for instance, you will use less material overall, with the HVLP unit, to get the final amount of finish that you desire on the item. The actual percentages that you get will depend on the items being sprayed. Spraying a line-up of chairs will be inherently less efficient than spraying a line of cabinet doors. Part of the issue is the psi at the spray head, which will cause less or more bounce-back. Individual techniques will affect efficiency to a degree. Just one instance: a wide fan will be less efficient (in terms finish-to-loss-ratio) than a narrow fan. In spite of what one shop owner adamantly claimed about his "95% efficient" rig, spraying spindle-back chairs will never come close to that number. Possibly using a brush could get you close. I set up a spreadsheet, based on his lacquer usage, to show how long it would take to recoup his $4000 cost for the rig, using the differences in efficiencies for HVLP and his new equipment. The answer: 10-12 years. For him, the purchase was closer to being a toy rather than a tool. Plus, there was a great salesman in that area. Having said all of the above, some systems may be more productive than others, regardless of efficiency.
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