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Yes, the pressure drop results in a corresponding drop of about 30 degrees. If there is moisture in the air being sprayed, or if the humidity is high in the surrounding air - that temperature drop will cause the moisture to condense in the finish being applied.

That may be the case here, but the OP stated he is spraying a satin finish - meaning solids have been added. Therefore, I cannot state the exact problem or required fix for this guitar without eyeballing it in person.

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"Act your age, not your shoe size" - Prince

Do you realize the cause of most blush is at the aircap ? It occurs at high RH because water from the atmospheric air gets sheared into and picked up by the atomization. I have never witnessed a condition where a coating sprayed fine and blushed on the surface as it dries. The thermal condensation thing doesn't hold either..... never saw condensation form on a guitar in my shop or spray booth like a toilet tank or window pane as that is caused by big temperature differences on opposite sides of the surface. That doesn't happen here.



I have lost count of all the guitars I have finished built by all manner of folks over the past decade and I have seen it many times on work as it comes in. ALso, I change paper more often than most anyone I have met and can still see he condition appear in front of me if I use a RO so I sand by hand.


Do you realize the cause of most blush is at the aircap ? It occurs at high RH because water from the atmospheric air gets sheared into and picked up by the atomization. I have never witnessed a condition where a coating sprayed fine and blushed on the surface as it dries. he thermal condensation thing doesn't hold either..... never saw condensation form on a guitar in my shop or spray booth like a toilet tank or window pane as that is caused by big temperature differences on opposite sides of the surface. That doesn't happen here.






Could be..... It's a vibration thing. Factories have less issue with this as most factory guitars are over braced. But a finer made instrument will vibrate freely and amplify those vibrations enough that RO sanding causes an issue. For example a RO running at 10,000 RPM will give an constant drive to the instrument of 160Hz, that's a constant E-3 note being played while you are sanding! Then there is the secondary frequency introduced by the random orbit which is slower producing a second, lessor constant input at somewhere around 30Hz or a B-0 note also constantly playing. So the top is vibrating, moving all the while you are trying to sand. Trying to work wood that is moving around never give optimum results.....

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Brian Howard wrote:
"I have never witnessed a condition where a coating sprayed fine and blushed on the surface as it dries. The thermal condensation thing doesn't hold either..... never saw condensation form on a guitar in my shop or spray booth like a toilet tank or window pane as that is caused by big temperature differences on opposite sides of the surface. That doesn't happen here."

When the finish has blushed in that fashion (outlining the braces) with the instruments I was finishing, the initial coating looked fine, but began to blush soon after I finished applying it. It seemed to occur most often if I moved the guitar body from a warm room to a cold one to spray it, and if caught early enough the instrument could be moved back into a warm environment and the blushing would go away.

I think I also recently had to deal with the stearates coming out of the (satin) lacquer when spraying a 9 foot circular "arch" out of doors. The temperature and humidity were less than ideal and it was Watco brushing lacquer. At first I thought it might be severe blushing, but the white patches felt soapy and scraped off easily. Not a fun day of finishing.

I could see that happening. I only started using an RO a couple years ago now and have not witnessed that problem but then I mostly French Polish too. But when using the RO on the top the guitar vibrates quite considerably for sure. In fact so much so that I was worried a bit about it. I figured who needs a Tonerite? Because using an RO will shake that thing in.

I guess the OP could solve this problem with one question, Did you use an RO on the top?

I strikes me that the cause is much more simple. The top is more stiff where there's a brace so more wood is removed when you sand. Sanding by hand is less aggressive so the braces show less. I've observed this effect of braces showing on the top only when the top requires some aggressive sanding like removing an accidental scratch etc. Therefore I'm very conscientious about keeping the solera and top clean when gluing braces or attaching the top/back to avoid any excessive sanding since I've worked so hard to achieve the desired thickness (stiffness).

This matches my experience I posted above. I was sanding by hand and was being too aggressive. I also assumed it was because of the added stiffness at the braces. The effect was diminished the further away from the X intersection and bridge plate I got. It served as a good lesson to be more mindful when I am sanding before finish. I always do it by hand so I was, perhaps, too complacent.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.