Yep. I'm sure others will post too, but this looks like classic blushing. It's water trapped in the paint. Happens when either it's humid when you paint or can also happen if it's hot and the dope surfaces dries before all the moisture can escape. Notice it doesn't happen over open bays.

Easiest fix is to shoot some thinner on the areas with a spray gun. This softens the paint and allows the moisture to escape. There are other methods that will probably get posted, too.

I always think of balsa as a glorified sponge. It seems only very old balsa stock is truly "dry".  I have this happen all the time and have just accepted it and it does go away as more dope is applied.  A taq (5%) of retarder in the clear helps also.

I did have retarder available, but never used it before when brushing.

Neville,

I often at least begin finishes in high humidity conditions. It's not uncommon for it to be 80-90% humidity in the shop during the rainy season (let's see, around here that's anytime but August and sometimes part of August). Never had problems with blush over open bays. Most common where covering overlaps since there is additional water there. I'm a bit surprised that Allan got blushing over Polyspan since it's applied dry. Balsa must have picked up some moisture in the humid conditions. As I noted, the only way to get rid of it is to allow the water to get out. This will happen eventually in any case as the dope gasses off, but it may take awhile. Applying thinner and/or retarder is usually the best way. Also, if you can de-humidify the room you are doing this in then apply thinner, it helps.

Doc, on really cold frosty days, the air can be very dry, to the point where I've had cyno that won't stick until I've moistened one side of the joint!

Cheers   

Well I have to disagree with the expert finishers that thought what I had was blushing, or trapped water vapor.    The reason I called it blotching was because it just didnt look like blushing.  Blushing has no grain or texture to it, just makes a painted surface look dull or hazy that would otherwise have a gloss.   I could tell that this was something that formed and was trapped on the surface of the balse and under the covering.  I could see it had a texture.    What I did today was take a mixture of 10oz straight thinner and 1oz retarder.  I wet the surface (pretty much flooded it) and watched and as soon as it started to dry (about 15 sec) I wet it again.  Repeated 3 times and at about the 45 sec point I could see the blotches start to melt away, or dissolve back into there original clear state.  I did the wing first, got rid of about 95% of the blotch.  Then I did the fuse.  Because it was flat I was able to flood it more so and kept it wet for almost 1.5 minute.  Completely cleared the blotches.  Then I noticed that there was some of the white stuff on the edges of the fuse where there was no covering, nothing to keep water vapor trapped.  If it was water it would have evaporated, it had been there almost 2 days now.  So I could now check it closer.  It was a powderey substance, and a little on the gummy side.  I could scrape it off with a hobby knife, or sand it off,  now that I had access to it.  One thing for sure, it was not water vapor, you can see that in the pictures..

Or some contaminate in the dope or thinner. Hmmm...

Congratulations, you found another finishing problem.   

Not to be a broken record, but it's advisable to use the recommended brand of thinner with dope. They are formulated to work together and sometimes other thinners have different components. It's not a quality issues so much as a compatibility issues. I've used Certified thinner with Randolph's and it came out OK, but I've also used Randolph's thinner with Certified dope and had some problems. Minor, but there. I've also used DuPont 3608 with Randolph's dope and had no problems, but with Certified, had some major adhesion problems. It's just better to use the recommended thinner with paint since you KNOW they will work together. It's one less thing to worry about.