Yes, You Can Have Too Much Ventilation - Popular Woodworking Magazine

Yes, You Can Have Too Much Ventilation

 In Chris Schwarz Blog, Woodworking Blogs


Earlier this year I decided to set up a spray booth in my shop so I could spray finishes on crappy days. It did not turn out well.

Usually I spray outside, so I have to wait for a pleasant day with the right humidity, no pollen, low bug count, no rain and etc. Sometimes the weather does not cooperate, so a makeshift spray booth seemed just the thing. So I bought tarps, filters and an industrial fan to ventilate things.

It seemed to be working really well. I wear a respirator when spraying, and when I’d remove my respirator when using the spray booth I was surprised how little solvent smell was in the air.

I was well-pleased.

My finishes, however, were not.

The fan moved so much air that my film finishes would become rough to the touch because of all the turbulence. It was similar to what happens when you spray too close to the work or spray over a wet spot two or three times by accident.

At first I thought I had the settings wrong on my gun. I did not. Then I thought I had the wrong needle size. Nope. Perhaps my turbine was losing power and couldn’t atomize the finish? Nein. My finish was old? Someone had put a handful of sand in the finish?

It was the air flow. Too much. Way too much.

I put the fan in the shed for now and have returned to spraying outside on suitable days. Maybe I’ll try again with a smaller fan, different filters or a different configuration. Or not.

— Christopher Schwarz

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Showing 14 comments
  • Liviticus

    Try a voltage reostat in the fan circuit to slow down the fan speed..

  • bearkatwood

    I know there has to be a joke about fans in there somewhere. “Is that your biggest fan?”
    Anywho, I have struggled with the fair-weather spray system for years and have considered a spray booth in the shop, but it has seemed daunting. Opting for the wipe on finish has been the go-to for me, but that might change and probably should. Thanks for the info on the ventilation Chris, much appreciated, and sorry for the cheese.
    Brian Noel

  • BLZeebub

    Easy enough to control the CFM or your chosen fan, just add a baffle or louvers or in any way constrict the opening and yur dun! But I’m with one of the earlier posts, your mix is off. I live in central FL where I have to contend with 100% humidity which leads to blushing if I lay it on too heavy. Even then, all I have to do is mist the finish with solvent and the blushing disappears.

    Like all things mastered in Woodville, we have to try try again. Think: micro-bevels and freehand sharpening.

  • pearlsb4swine

    The goal should be to engineer an air velocity of 100 feet per minute in the area where you’re actually spraying. Note that that’s a different measurement than the total cubic feet per minute that the fan is moving.

    • sgunsel

      100 fpm is usually excessive, but unlikely to cause problems in itself. It is a stone age ventilation rate that is OK but no longer necessary (see NFPA 33).

      Chris, come on…if you are spraying a liquid, it has volatiles. Were you powder coating?

      • pearlsb4swine

        NFPA (National Fire Protection Association) 33 “This standard provides requirements to mitigate fire and explosion hazards of spray application processes that use flammable or combustible materials”

        100 fpm might be more than enough for safety but we’re discussing quality of the finish.

  • sgunsel

    I doubt turbulence is the real culprit. Although not many relevant details are provided, I suspect the real problem is improper solvents in the coating material for the application conditions. A “retarder” such as used for use in high temperatures might help. The increased airflow, assuming all other conditions such as temperature and humidity are the same, could be causing solvents to evaporate at higher rates, especially the more volatile ones. I would contact the coating manufacturer to discuss.

    • Buildinggeek

      Bingo, correct answer! Setting up a spray booth without knowing the airflow rates (not the fan “rated flow”) is a total crapshoot. Measuring airflow can be tricky, but if you have any local friends/acquaintances in the home performance business they should be able to help you out.
      I think you’re probably closer to success than you think, you just need to understand the airflow better.

  • Phil Spencer
    Phil Spencer

    Should be careful Chris using a standard fan and motor when spraying what could be volatile substances. I don’t know what the standard in the USA is but in Australia hazardous area motors have to be used usually rated EeXD or EeXE explosion proof and non sparking and a case strong enough to contain an explosion if the motor goes BOOM through over heating or a spark. Same goes for heating a cold workshop, wood dust can be combustible and an intrinsically safe heater should be used, Ie… something without exposed red hot elements Oil Coulomb heaters are good as there is no exposed elements .
    This type of safety might be a good discussion point.

    • Christopher Schwarz
      Christopher Schwarz

      Did I say I was spraying volatiles?

      • Phil Spencer
        Phil Spencer

        Just read a post on FB about a fellow who left a rag soaked in BLO on his bench overnight it spontaneously combusted, you may not have been spraying volatiles Chris but your post did not say this. Isn’t it best to err on the side of caution? I still think where workshop safety is concerned one cannot talk enough about it or safe working practices 🙂

  • Matt_Rob

    I have found that with my latest project a small table that adhering to the man code and neglecting to read the application instructions on deep red pigment water based stain Black cherry General Finish followed by spraying Minwax water based poly finish . The result was a pink top coat. Fail . You can mix empire and mission style and get the stink eye but top coat and wax you get the fish eye.

  • zackdog

    In our very nicely appointed industrial chemistry lab, our fume hoods move about 700 CFM – that classifies as Class A (which is very good). And the air in our lab changes about six times per hour and there is never a chemical smell (but lots of chemicals present). Perhaps you could engineer your a flow to be in that vicinity (500-ish?).
    My advice is go for a laminar flow situation and pull the fresh air into the “tent” through a couple of different ports. And if you’re wearing a respirator for personal protection anyway, you shouldn’t have to over-engineer the spray tent.


  • Dave in Ohio

    It would appear that you need a Little Ass fan. They are quite popular, as I’m sure you have heard the advertising slogan, “Everybody likes a …………………………………..”

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