Humidity and finishing

[BWSmith]

We have climate controll in shop.......can be set for temp and humidity.

Our spraybooth;24x26 was built with the intentions of being able to paint cars.Personal use,not as a "biz"........about 20 years agver that time.....only a cpl cars went through there,great results.Yes,we could use a little more HP on the admittedly,hillbilly setup.

Fst fwd'ing........Am trying to wrap my pea brain around humidity in there WRT spraying furniture/cabmets.Have paid way too many dues in "brush" techniques.......and still love doing brushed finishes,but take that out of the issue.This about spraying.

I understand some mis-conceptions about relative humidity.........so the question is really how can I lower the absolute humidity as cheaply as possible.Right now am playing "tag" with it and spraying early in a.m. and allowing finishes to flash-off and then relocating them back into AC'd part of shop.

How does moving air affect humidity?Any ideas or notions on the subject are appreciated.BW

 

[SteveHall]

Unfortunately, removing humidity is more expensive than cooling. The process of refrigeration does both. But to lower humidity (RH) in already comfortable air, an additional energy consuming re-heating process must be added. (I'm assuming something like a basement dehumidifier doesn't help you enough, these are effective when temp-RH are very high but less effective in getting to where I think you are aiming, say 74F/35%RH.)

For example: On mild days in a two AC unit house, turn one on cool and the other on heat for an hour, reverse them the next hour, continuing to reverse them hourly throughout the day. The RH will be significantly lowered without over-cooling the space... but at great expense!

Humidity is not as much an issue on hot days since hot air won't hold as much humidity and the A/C runs enough to lower the RH. But it's when temperatures are mild that humidity control is difficult. That's when the air feels "sticky." Sadly, there are lots of days like this in eastern NC. Even on super hot days, the nights cool down to where the AC won't run enough to lower the humidity.

I'd trade hot dry air for cool wet air any day. 40% RH at 95F is much more comfortable than 70% RH at 80F. According to the ol' Pyschrometric Chart, both have the same amount of energy per cubic foot. But the latter is much less comfortable for humans and woodworking finishes, and requires more energy to temper.

If it were me, I'd first try to seal off the finishing space as much as possible. Any tiny crack or gap will let in more humidity than you can control. Without a vapor barrier, sheathing, siding, concrete slabs, and drywall all allow considerable humidity to infiltrate, too. Also minimize the time doors are open to convening spaces as much as possible.

Then, after completely sealing the space, I'd try running the AC non-stop for 24 hours prior to finishing while using a small heater to re-warm the air for comfort and to avoid condensation (lowering the temperature to dew point). If you're only finishing occasionally it might add only a few dollars to the energy bill each time.

Lastly, I'd recommend getting one of those weather stations that measures both exterior and interior temp and RH. Maybe chart these for a few days on the hour to get a rough feel for what is actually happening in your shop. You'll be surprised at how high the RH gets in the mornings inside before the AC comes on! It will also help to understand the effect of the measures you are taking.

 

[Tarhead]

You're right about RH misconceptions. Relative and Absolute Humidity are like Apples and Pineapples. Dew Point (Wet Bulb) is a much better way to keep track of humidity in the space. What's outside and inside are different too.

You gotta get rid of the water vapor if you want to reduce it. Run a dehumidifier and a fan to keep the space from having dry and moist areas. Just a fan by itself won't remove water vapor. It just moves it around.

 

[BWSmith]

We were at the dew point this a.m........64* and 96% humidity.And,sure enough,needed to be out there spraying.My friends have been telling me repeatedly about insulating and in general,sealing up the space......and I keep resisting.Its the money.IF,there was more spraying out there to justify it there wouldn't be a problem of $$.And generally have been using WB finishes in the main shop,only heading out there on high VOC projects.But that(finishing in shop)isn't without other problems....duh.

Have been doing lots of research,this site has some very good info.......along with google.Thanks for the interest,BW

 

[Gofor]

With the high VOC coatings, you are concerned with the relative humidity and dew point. The most common way to get down to an acceptable RH is to raise the temp, but with a high dew point, that may mean raising the temp above a suitable level for the coating to dry correctly. However, with the 64 degrees & 95% RH, you probably could have made it work by upping the booth temp to 80 or 85 degrees.

The main problem with high humidity and spraying is caused when the compressed air expands as it leaves the air cap on the spray gun. As the air expands, it cools the surrounding air. With high VOC solvents, the rapid evaporation of the solvent when the gun atomizes it during spraying adds to the cooling effect. During high relative humidity, this cooling will cause the surrounding air to drop below the dew point, at which time the moisture in the air condenses into water droplets that then become entrapped in the coating.

With lacquer, this is called blushing and shows up as a milky, cloudiness in the coating. One way
of reducing blushing is to use a blush retardant thinner. The trade-off is that the lacquer will dry slower to give the moisture a chance to evaporate before the coating dries which also gives the coat more chance to run or sag, so you have to apply lighter coats and extend times between coats. This method does not work well with lacquers that have metallic pigment in them, because the metal flakes will separate out before the coating cures.

With enamels, varnishes and polyurethanes (wood coating type), the moisture entrapment will either stop the coat from curing, or may leave craters in the surface as the water droplet floats to the top as the coating is tacking up. Using a slow thinner like mineral spirits works better than one that has ketones added to hasten the solvent evaporation.

Using as low of an atomizing air pressure as possible, spraying light coats, and using slow drying thinners will let you push the envelope on conditions quite a ways. If you do plug most of the air gaps in the booth and use heat or dehumidification, a cheap way to keep track of the booth conditions is one of the $25 atomic clocks that has and outdoor temp/humidity sensor.

Go

PS Hope some of this info helps. It is based on quite a few years spray painting aircraft, sometimes in a hangar, rarely with climate control, and work stoppage due to temp/humidity was rarely an option if it was above freezing.

 

[stave]

This may sound strange but I have found that it works really well for me. I spray lacquers in a small shop without a spray booth. Of course I have to turn the heat off in the winter while I spray. Blushing was always a problem and not only with lacquer as I also occasionally use water based and poly finishes.

I heat up my finish before I use it. I put a container of water in the microwave and get it to almost the boiling point. I then fill my spray cup with finish and put it in the container of water, basically a double boiler technique. After a few minutes I dry off the cup and start to spray. This has saved me numerous times on those days when it is raining and the rush job has to get done.

Good luck

Stave

 

[BWSmith]

Good stuff there guys!

Well,its raining right now......garden's happy but spraybooth ain't.

Being only one step above computer illiterate,will work on pics.I built the trusses in there and along with a rather well thought out overall design it works pretty darn good.The only real "gaps" are because of barn style tracked doors.....otherwise its basically "sealed".A cpl dehumidifiers would get moisture in check.Understanding explosive nature of fumes and overspray,is there a problem runnin dehumidifiers in the presence of fumes?

A little detour,but related.........a cpl years ago was trying to come up with a moisture "trap" for our compressor.Did the "fruitjar" and disposable filters on gun.......but was wanting to step up the whole dry air thing on compressor.Here again,pics forthcoming,but anyway we Tigged up a very nice water cooled "pre-chamber" that got mounted inline between compressor head and the tank.It's effect is it drastically cools the extremely hot,moisture laden air before it reaches storage tank(2 stage,60g).Painted to match the rest of compressor,it works better than we even imagined.Shooting temp gun at the before's and after's.Its main "trick" is that it cools and loses moisture BEFORE tank instead of treating the problem after the fact.

So,it was in this whole,cheap.....that makes too much sense way,that I'm looking for ideas on moisture within spraybooth.Thanks for the interest,BW

 

[jmauldin]

I second what Gofor said. Very good response, and "right on the money"
Jim in Mayberry

 

[BWSmith]

See if this link works,interesting reading.

http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html

Back to the "detour" mentioned above.Part of the process in the pre-chamber is mechanical in the sense that water vapor is heavier than air.......so the cooling affect of this chamber only represents a sm part of what's going on.The more important part is a "re-direct" of the incoming air.The water droplets or vapor are being slammed against the cold,chamber's walls and forced to take a specified or predicted path.The air,being lighter takes a different path......away from water/oil vapor and heads to compressor tank.

Have been using a search function of one of the larger HVAC forums.......still researching.