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Hey,
Has anybody ever had problems with blushing while spraying nitro? I've been careful to only spray between 40 and 50% but I'm just wondering at what point you might start to see problems...
cheers,
Jordan

Well... I don't recommend it, but I sprayed a guitar the winter before last, when there was 4" of wet snow on the ground. This was before I finished my spray room, so the spraying was done outside.
Maybe I'm just lucky, but I haven't had any blush problems with lacquer yet.
It's my understanding that blush shows up pretty quickly. If you are concerned, maybe try a test panel first.
I usually thin my lacquer 10 - 15 %. Maybe that's why I haven't had an issue with it.

For high humidity (anything over 60% will cause problems with blushing) add some retarder or Butyl Cellusolve to your lacquer before spraying, to slow the dry time in order to allow moisture to escape. I added a little more retarder to my lacquer mix with spraying in Taiwan because the humidity is persistently over 80% except for some rare circumstances. I sprayed it during the monsoon (where humidity is in the high 90's) and no blushing at all.

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I sprayed out doors for a number of years and don't recall ever having a problem with blushing. I know I've sprayed over 50 without problems. Like Daniel I also thin my Lacquer a bit. And like Daniel I've also sprayed out doors in the winter, only I had 4ft of snow! I don't recommend it ( where I live it can be a very cold process) but in a pinch it does work. I've found Nitro to be very forgiving when sprayed under a variety of conditions.

Josh

Usually you'll be OK if the temperature (F) is 15 higher than the RH%.

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I should have mentioned that I use a standard, piston type compressor but I drain the tank regularly & use a large, good quality air filter / water trap. I think you're more likely to have blush problems with contaminated compressed air, than with high humidity.

I have use an oil filled electric radiator with good results to heat the lacquer and gun. Just turn the radiator on low to medium, set the gun between the "fins" for a few minutes...

I've experienced "blush" exactly once -- it was horrible and I freaked out, but more on that later. As a result of the experience, and being a scientific type guy, I did a bunch of research to understand what causes blush and how to manage the risk.

When a liquid is atomised under pressure, its temperature drops. If the temperature drops to the point that water in the air condenses on the atmomised droplets, that water is drawn into the finish and can appear as an awful whitish cloud -- blush. This can happen (as it did in my case) with no water at all in the compressor system.

To avoid blush, many internet resources refer finishers to the 60-60 rule, meaning "spray only when the air temperature is above 60°F and the relative humidity is below 60%". After monitoring conditions closely during subsequent spray sessions, I would agree that adherence to this rule is probably very effective at avoiding blush -- but in my location, I wouldn't be spraying very often!

The parameter we actually want to monitor is "dew point", the temperature at which water vapour condenses at constant pressure. If our atomised spray remains above this temperature, water won't condense on the droplets, and we won't get blush. To calculate "dew point", you need observations of ambient temperature and humidity in the air supplied to your spray booth, and the atmospheric pressure. You can find formulae by looking up "dew point" in Wikipedia, or you can do like I did, simply install an automatic weather station that sends temperature, RH and barometer data to your computer and calculates dew point for you. I now only spray when the dew point is at least 10F° (6C°) lower than the ambient air temperature.

So, what did I do when I saw blush, besides freaking out -- it was during the final coats and I had visions of scraping and respraying the instrumment. I live in an area where we are prone to blush conditions, so when I called up my coatings tech rep for advice, he was quite calm in his response: "Just wait a few days for drier weather, then respray -- the blush will disappear". He was right, it did.

Others have mentioned the use of slower thinners, retarders, and other additives to help the water escape before the film flashes. Excessive retardation can affect the cure and I'd only use this approach if I was forced to spray under poor conditions. Also, warming the liquid (hot potting) is pretty common (use common sense, as the liquid may be highly flammable and explosive), and will help with both blush avoidance and levelling. I use a piston compressor and HVLP conversion gun, but turbine systems are probably superior and they warm the air, thus discouraging blush.

what I do is to mix up the lacquer, put it in the cup (siphon cup) attach the gun to cup, and then place the cup into a pot of hot water (from the tap)...let it sit there for a few minutes and start spraying...