Official Luthiers Forum!

I have applied 6 coats of Watco rattle can lacquer to my #2 with good results so far. However, this evening I sprayed a coat which immediately turned cloudy in a weird pattern which follows the bracing pattern on the top and bottom. I can scrape off the clouding with my fingernail (see pics) The top is bear claw sitka and the back is padauk. See PICS. WTH I'm freaking out!!

Here are some of the factors...
- I keep the guitar and neck in a very low humidity shop < 30%
- I bring them outside just before I spray, then I spray, let sit for 5 minutes and return to very dry shop. This has not caused a problem so far
- It has been cool 40 - 45 Deg F ish for the last 4 coats but as I said I return the guitar to a warm dry shop shortly after spraying and has not been a problem.
- I do not store the cans outside.
- Yesterday was warmer 55 deg F and I applied 2 thicker coats two hours apart. It was looking great!!
- Just before I sprayed this coat I sanded with new 400 grit sand paper from home depot. I used 320 on the previous coats.
- Today It rained and was still fairly damp and 50 deg F when I sprayed a fairly heavy coat which resulted in the problem.

I'm baffled and really deflated. This guitar was turning out really nice. Help!!

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Alain
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Cold and damp (read RH is 80-90%) .. sprayed a heavy coat .. thats easy .. you have some serious blushing going on (trapped moisture in the finish coat) .. it may resolve itself over time as the vapour escapes .. if not, sand it off and spray lighter coat in drier conditions.

Q - How are you keeping your shop below 30% RH in Ottawa at this time of year ? I am outside Toronto and even with a dehumidifier going full blast, I can only get to 45-47 ... needless to say no bracing going on just yet - I like it at 40-42. If thats what your hygrometer is saying I would have it checked out ... whats the reading from the local airport/weather stn ???

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Too bad. That guitar is ruined . . .

No, it's just nitro blush. Guessing its fairly cool and humid in your local. (Reading your post further confirms this).

First, stop scratchin' the finish. Second, take a deep breath. Then you have a few options. Spray a mist coat of lacquer thinner with a small amount of retarder added. Behlens sells a rattle can blush remover with the same properties if that's easier. That will open up the surface film to let moisture out. Or you can probably just wait till the air is drier and keep spraying. Blush generally goes away this way. I suppose the third option is to sand it of the surface, but that wouldnt be my first choice. If humidity is greater than 50% and temps less than 60 degrees, don't spray without some retarder (but use sparingly and try to avoid if possible because it drastically slows overall cure time).

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Aaron Craig

My shop is only 8' X 12". I have a small dehumidifier going constantly.



How about if I wipe it with a paper towel lightly dampened with lacquer thinner?

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Alain
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Also. Why would it follow the bracing pattern?

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Alain
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I just cant see wiping working, lightly sand and spray a light coat of thinner with some retarder in it, it should go away.

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Jim Watts
http://jameswattsguitars.com

Wiping with a paper towel and thinner sounds like a bad idea, but I don't know for sure since I've never tried it. Go buy behlans blush remover since it doesn't look like you have spray equipment. Scuff sanding before hand is a good idea if its been a bit since the last coat.

Not sure why blush tends to not occur over braces, but it generally doesn't. Maybe it's an insulation type effect?

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Aaron Craig

You brought the guitar from the warm shop out to the cold. While spraying the top could get below the critical temperature, except where the braces kept it for long enough above this critical temperature (the brace's mass means stored heat). This slightly warmer top surface above the braces (and bridge patch) was enough to keep RH of the "air layer" above the braces low enough to not allow blushing.

Call it a feature and charge accordingly more!

If you can scratch the blush off with your fingernail I'd rather sand it back (just like Jim suggested) than try to wipe it off with.

Going from warm to cold with humidity present will cause vapor to condense on the surface. The bracing retains warm longer due to its mass. Chances are you brought the guitar outside and the thin surfaces acclimated to the colder temps before much condensation occurred. The warmer areas over the bracing continued to promote condensation and were not completely dry when you sprayed.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Wiping with a thinner soaked paper towel will make a mess of the good coats of lacquer you have already sprayed. The moisture is only trapped in the last coat sprayed so sanding it off may be your best option. Blush remover (retarder) can also work, but if you don't have it handy sandpaper is your friend. If you are spraying outside, allowing a few minutes for the guitar body to reach equilibrium with it's surroundings will help reduce blushing. If you do notice blushing you can sometimes minimize or eliminate it by moving the piece to a warmer, dryer place immediately.

I can't spray the nitro mix I use when the RH is below 60%, or I get blooming. The temperature doesn't seem to matter. The thicker the coat you spray, the greater the risk of blooming. As the thinners vaporises, that takes heat (the latent heat of vaporisation) which cools the wood as others have mentioned, less so where the wood is thicker because there is more heat to give. Once the wood surface temperature drops below the dew point, you get condensation, AKA blooming. If you can get the piece somewhere warmer and dryer rapidly, often the bloom will clear, as Clay said. If you spray over, sometimes it will clear. If you spray over and it doesn't clear, it seems to "lock in" and sanding is the best way out.

That's not my experience when it is cooler outside. The piece just drops in temperature to the dew point faster.

Best solution for me is not to spray unless its below 60% RH in the spraying environment.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

The same thing happened to me on one of my last guitars. Get the Behlens blush remover in a can, spray a very, very light must of it on and watch the blush fade away. I was able to spray another coat within 10 minutes. Worked perfectly and the finished product look like there were never any issues. Trust me. Don't bother sanding, just get the blush remover.


Posted from my Mustache using Tapatalk

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http://www.CostaGuitars.com
PMoMC

When I am spraying in cold weather the lacquer is kept in the same cold conditions. It behaves a bit like lacquer with retarder added - the thinner doesn't volatilize as quickly, so perhaps this is why it doesn't seem to blush as much. The "art" of finishing is finding out what works for you.

Sorry, that should have said:

"I can't spray the nitro mix I use unless the RH is below 60%, or I get blooming".

Seems people read what I intended though! (or everyone is very confused!)

Fortunately, around here, if you don't like the weather, you just have to wait a couple of hours.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

interesting...

a trick I was taught years ago (and one which has worked very nicely over time) is that when spraying in cold conditions to take your product once it is in it's spray cup and put that in a pot of warm water to pre-heat it...take out of pot, wipe off dripping water, and then proceed to spray...as noted, that has worked many times for me with no adverse affects...well, there was that one time I did get a bit of blushing, but I didn't realize the outside temperature had dropped to below freezing (I thought it was still in the 40's F)

obviously it is best to spray when conditions are "right", but sometimes one has to do what one has to do...

Pardon me......totally backwards....

Taking the guitar from a colder room outside to warmer air with moisture present COULD create the situation you describe....the areas with mass requiring more time to acclimate. Sorry for the confusion....I stay up way too late sometimes.

You'll need a chemical solution to this problem....no doubt. I was just interested in the fact that the blushing seems to track or outline the bracing and I tossed out a theory. But I would add that when processing chemicals, the best course is to eliminate all the variables that you can. It will be ultimately more difficult to account and compensate for temperature and humidity issues by altering your chemical ratios than it will be to control the temperature and humidity of your spraying environment.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

A friend of mine, who is a very good finisher, uses the hot cup technique in cooler weather. I believe he does it to improve atomization rather than to keep down blushing.

With lacquer, temperature is not nearly as important as RH. I have sprayed lacquer when the temp was 15 degrees F. In the middle of winter, a cold day may actually be the best opportunity to have low RH.
If the blushing is not severe, I just spray more lacquer....unless sanding is called for in my schedule. In general, I spray 4-5 coats a day, and level sand with 220 at the beginning of the second day.
I have sprayed straight retarder on the lacquer to get rid of blushing. Mohawk aerosol Blush Eraser probably is a similar product, if not exactly the same.
On humid days, I just add more retarder to the lacquer.

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John

Thank you all for your great advice.

BTW: I can't find any kind of blush remover here in Canada and I don't have enough patience to wait for a shipment.

By the end of today, the finish was dry enough to sand. However, in my haste I ended up sanding down to the bare wood in a few spots and there's more sanding to do.
My question is: now what?

- Should I sand the rest of the finish off and start over

OR

- Should I leave the existing nitro and apply vinyl sealer over the whole surface; will it stick OK to the existing nitro as well as the bare wood; will it look uneven?

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Alain
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I would just remove all the sealer and start over. The possibility of visible sand-through is one reason I gave up on using vinyl sealer (or any type of sealer). I just use lacquer.

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John