I'm blushing

It rained here yesterday, but today dawned nice and sunny, so I decided to spray a sealer coat of freshly mixed shellac on my recently-grained-filled tenor. Must have been too humid, because the shellac turned white in quite a few places. So I've ordered a hygrometer. Which leads me to two questions:

1. Is there a "tipping point" in terms of relative humidity at which blushing becomes a problem?

2. Can lacquer retarder or some other substance be added to the shellac to prevent blushing?

Thanks in advance.

Michael

The RH where I live commonly ranges between 65% and 85% so I'm all too familiar with blushing as it pertains to nitro. Spraying in a warm and humid climate is problematic but many of us do it successfully.
The problem is you're trapping moisture in the finish and the skin of the coat has dried before it has a chance to escape. Thinning will help, partly because you're applying a thinner coat and less material has to dry. Certainly a bit of retarder (a very slow drying thinner) is helpful in slowing the drying down so that the moisture can evaporate. Spraying coats before the previous one has had a chance to dry can also cause blushing. You might even experiment with different brands if it continues to be a problem. When I was spraying McFadden, I could spray any time the RH fell below 76%. With the Mowhawk I need to wait until it gets to 72% or it'll blush.
It's a good idea to have a humidity meter in your spraying room. It's the only thing I rely on to tell me when I can spray. You might also try spraying later on in the day (mid-afternoon) when the RH is naturally lower. And never spray when it's raining or when you think it will soon.
If it's a bad case of blushing, usually waiting until the RH is lower and shooting a coat of solvent over the instrument will make it disappear. If it's only a slight case of blushing, most of the time you can simply apply another coat on top of it when the conditions are more favorable. You can also sand it out but I've never found that necessary.
Hopefully Allen McF can jump in. I have a feeling he's an expert on the topic.

Blushing in shellac I've only seen a few times and it was always when it was sprayed.

As Chuck said, it's trapping moisture under a skin that is formed that causes it. My expertise is in spray-painting in the automotive trade. Been at it for over 30 years now. But we don't use a lot of shellac there.

I very well could stand to be corrected here, but one source of water / moisture in shellac is the alcohol itself. A lot of alcohol that you get is not 100% pure, and even those that started out that way will absorb water if left open. Thinking of the cheap hardware store variety that we get here in Australia. Some is so bad that shellac won't even dissolve in it.

The next area of possible moisture contamination comes from your compressor. It's the nature of the beast, and you need to have a good water / oil separator installed at the very minimum of 3 meters down line from the compressor or you're just waisting your time. Then finally there is the humidity that is in the air when you are spraying. This is a big one for most people, as the other ones can usually be dealt with. The only thing that you can do is wait for better conditions.

When it comes to spraying shellac, I don't know of a lot of things that you can do to stop blushing other than using 100% alcohol. You can't use any sort of retarder that I'm aware of, as they aren't compatible with shellac. And spray when conditions are optimum.

Better yet, just wipe it on with a rag in very thin coats and you should be good.

One good thing about blushing in both shellac and lacquer though is that they are easily repaired. For the shellac, you can come back with another thin coat when conditions are better and all should be good. I've also just did a wipe over with some shellac on a rag, over the affected areas and they disappear.

With lacquer always add extra slow thinner (retarder) to the mix. Never more than 10%. This will keep the surface open, help with flow out, and you'll use a lot less material to boot.

What Allen said. And remember, with shellac less is more.

We use isopropyl that is declared to be 99.8% and haven't had any blushing problems with it so far. Also, we only polish in really good weather.

We did have some blushing on our very first polishing job, but that was made with cheap hardware store alcohol and I suspect we also layed it on a little too thick. It did rub out afterwards, but I seem to recall that quite a lot of elbow grease went into it - not to mention the panic as it was the day before a tour.