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I have retarder and thinner (Mohawk). Ratio plz? Also, it seems to me that pressure at nozzle matters. Thoughts? Walcom EGO HVLP gun.

Um, I think found the problem. I was using retarder, but i didn’t have my line dryer in place lol

oops

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Steve Smith
"Music is what feelings sound like"

Think I’m going to buy a dedicated line dryer. I would like one with replaceable desiccant. Two stage if possible. Thoughts?

I will probably add an in-line take off port for construction guns. But when I’m spraying I’ll use the dry channel

Hi Mike, I don't know exactly what the climate is in Huntsville Al. , but let me tell you it's hot and sticky here in Soggy Bottom U.S.A., I've been finishing one and I'm getting "blooming" also, that is a haziness in the last coat applied. Dry air in the line is a good thing but it won't stop it . My rig is a Fugi 3-stage turbine and I know that air is dry but the problem still exists. I just wait till after noon when the humidity drops below 65% and go for it. If you get it again just spray (rattle can) it on the instrument straight from the can. I used to have just the liquid retarder and would use about 50/50 with thinner. I can't find that anymore. Where did you get it (the liquid retarder)? Oh yea, and the humidity here went from 91% this A.M at 8:00 to 50% right now (5:45P.M.).

I use an inline set up also run dehumidifiers in the spray booth

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

If I start to get blushing I immediately place the piece under incandescent hot lights and that helps drive off the moisture. Just don't get things too hot.

Dry air is half the solution. Old automotive painter’s rule of thumb: temperature above 60°, RH below 60%. The more correct spec is dew-point difference from ambient temp greater than 5°.
Anything different, you’d better be using retarder and “hot potting”


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I’ve noticed a pattern. Seems I can go 3-5 coats then things go South. In-line dryer, retarder (stew Mac). So I’m gonna wait till tomorrow to do last coats. Body and neck look awesome now. After scuffing off blush. Seems in-air humidity is the bogey man

John and Tim's posts have key information. Spraying finish in uncontrolled hum./temp. conditions is problematic for sure,
especially hot muggy summertime conditions. If the conditions at exact spray location can't be controlled, a workaround for
me that has never let me down is this... guitar is hung in drier and warmer conditions than spray area, quickly taken to spray
and returned to hang in dry/warm area straight away.
As blush is moisture being picked up from the air, not unlike condensation, you can mitigate it by similar techniques that will prevent
dew and condensation.

if you are going to spray Nitro get a dehumidifier
I set mine at 45% when spraying
I use a fuji turbine and I have hvlp air compressor sprayer ( saga and some HF cheepies )
i use a mix of 50 50 thinner and retarder and mix about 50 50 to 60 40 depending on the rh and don't have bloom issues finish/ret thin
I also use a turning holder
https://www.youtube.com/watch?v=escxSMDoC5g

I use a down draft spray table and my spray booth is about 6 by 8
I have 3 vent fans
8 in to pull the down draft on the spray table

2 4 in fans
one is set up to pull VOC's off the floor ( both 4 in fans are 100 cu ft min )
the 8 in is 560 Cf Ft min
I have a fresh air vent to allow fresh air into the spray room . My rh stays under 55%
I also have a heater I can set to 90F
I can hit a coat an hour and do 6 to 8 coats I use a 1.0 mm needle in my fugi and I run the 8 in fan 5 min after spray to pull fumes the 2 4 in fans run all day till I am done spraying to pull fumes and VOC's.
If you can control the booth you eliminate a lot of issues

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I use MOHAWK (Behlen) Blush Eraser in a rattle can to get rid of blush. Disappears right before your eyes, for the most part. You just mist it on from 12 inches or so, just a mist, DONT LAY IT ON. Mist falls on the lacquer and releases the water vapor (solvent melts and retards the lacquer, allowing the water to escape. Works at time of spray or weeks afterward. I don't know how it would work if you sprayed lacquer over an existing blush, but its like magic on the most recent coat. Sometimes I mist it on just for insurance (I'm sure this is not a best practice, but neither is spraying outside...). THank you for the other tips on reversing the blush too.

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Measure Twice,

Karl Borum

I've been spraying nitro in Hawaii for the last 40 years and before that in Washington State. Both with very high humidity issues. Chemicals are a difficult problem for us here. Getting anything special is VERY expensive. The last time I tried to get anything special, it was Cardinal and 4 gallons cost me $500.00, after the Hazmat! Anyway, I have a lot of trouble with blushing, using the only nitro I can get, which is Sherwin Williams. My available spray days are minimized by the weather. I have a heated cabinet for warming the instruments and I draw dry air through the booth from my 45% dehumidified shop. The only retarder available to me is Sherwin Williams Retarder Thinner. It does not do much more that regular thinner to prevent blushing. I've heard of people using other chemicals as retarders, like Butyl Cellusolve and Xylene. Any experiences things not actually labeled as retarder for nitro? Thanks, Bob

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“ The meaning of life is to find your gift and the purpose of life is to give it away” Pablo Picasso

A builder from this area (Carol Benoit) had/has the best set up I've seen. He has one end of his shop for finishing and has this divided into 2 rooms. One has a dehumidifier and adjacent to it is the "spray" room with an intake with filter from the dehumidified room and an exhaust fan for fumes. The turbine compressor is kept in the "dehumidified" room which supplies air for the spray area. That way he sprays in a "dry" space.

Interesting. Wouldn’t the dehumidified air be exhausted pretty quickly? Could a dehumidifier keep up with ambient air coming into the “dry” space before its then pulled into the spray space?

My setup is a booth in the shop and then two windows open on the opposite side of the space to draw in fresh air. The humidity goes up 1–2% per 10 minutes the exhaust is running until it eventually levels out pretty close to the outside humidity level. So I have to still watch outside humidity (granted, the material I use is more forgiving than nitro in this aspect).




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Clay, what perfect timing. After reading the thread yesterday, I sprayed Nitro at 53% RH outside.
Really bad blush on the rosewood back, seldom happens to me at below 65%
.
Put an incandescent light on it , the blush completely reversed after maybe a hour.

Interesting: after the light bulb treatment, everywhere it was blushed turned into "dust" like overspray. Wiped off with a soft cloth. Wouldn't photograph or I would have included a picture. Pretty strange.

Thanks for the warm light bulb trick!

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Measure Twice,

Karl Borum

With solvent-cure finishes, you generally don’t have to do anything special to get rid of blush — heat treatments, blush eraser, sanding, etc. are not really necessary. If it happens, it is a sure sign that water is entering the finish either from (a) the air supply, or (b) condensation on the cooled droplets (dew point too close to ambient temperature). Correct the problem(s) and the blush will disappear when you recoat under proper conditions. (Technical advice given to me by Mirotone when I was panicking about my first experience with blush.)


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I can't recall any time I got a dusty surface, but there have been a few times the blush didn't completely disappear and I had to scuff sand to remove the last of it. Glad it worked out for you.
When I think conditions are right for blushing I will set up some lights and put the item under the lights before I see any blushing occuring. The Mohawk No Blush/ Retarder works good but I avoid it if I can. It does slow the drying down some.
Tim is right, a better setup might eliminate blush problems without resorting to heat or chemicals, but at my present "production level" I'm not sure the additional expense would be worth it.

So I studied up on relative humidity and dew point. Maybe I'll buy one of those sensors. What else I have noticed is that the problem becomes pronounced as I move into the evening hours. This makes sense. Temperature drops, approaches dew point and bam. Blush. Its not a horrible problem unless you keep spraying over it. A scuff sanding gets rid of it. And, wait till the next day. I would like a more sophisticated setup, but for now, I'll manage it with schedule.

A couple of notes:
1. When I was building in NZ, nitro was unavailable, so like most builders there, I used a pre-cat polyester from Mirotone (Australia). I had some initial application issues and got technical advice to use a slower thinner. Mirotone makes at least 4, ranging from fast to super slow. That’s where I learned the importance of reducer “speed” on flow out, as well as blush control. NZ was humid and cool, so ripe for blush.

2. Now in Canada, I went with Mohawk nitro and was surprised to learn they only have one reducer and a bunch of additives. I had flowout problems and a well-known Canadian builder recommended a very small amount of their Super Blush Retarder (which is simply straight butyl cellosolve). He also addressed my “slow build” complaint for the Mohawk Classic Instrument Lacquer by recommending the Mohawk Piano Lacquer with higher solids. That just left the problem of super-cold supply air for most of the year!

I made the switch last year to outsourcing my finishing. Not cheap, but much less stressful and the best decision I could have made. Blush and many other issues are a thing of the last.

The piano lacquer has less plasticizer in it so is more prone to "checking" when applied to thin materials like guitar plates, but some people like that look.

I have no idea if that’s true or not. What I DO know, is that my friend is selling his guitars to discriminating players around the planet, and they sing his praises (and come back even when the price has gone up).


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Tim- What was the proportion quantity of that "very small amount of butyl cellusolve"?--Bob

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“ The meaning of life is to find your gift and the purpose of life is to give it away” Pablo Picasso

Hey Bob,
Yes, an important detail, but I wasn't in my shop when I posted! Remember that my using a retarder like butyl cellosolve was intended to improve flow, rather than avoiding blush. Mohawk only sells one reducer, and it is "fast".

Add 10-20 ml butyl cellosolve per litre (1 to 2%) of Mohawk nitro that has been thinned up to 15% with reducer. Many folks describe using much higher proportions of reducer. I was advised not to do that when adding butyl cellosolve, and not to exceed 20 ml / litre of mix. You only want as much reducer as is required with your spray system -- some can spray Mohawk straight. Every product is different in subtle but important ways.