Exterior Paint bubbles

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In the fall, when painting conditions were just about ideal, I painted the exterior of two walls of my house. It took 2 coats a few days apart to cover nicely and it looked really good. A few of days ago, I noticed a couple of blister like bubbles about 1-2 inches in diameter at sort of random spots on the walls. The old paint underneath was in pretty good shape, I primed any peeled spots, the coverage seemed good, I had sanded the old paint with 60 grit orbital sander, washed and dried the walls good before painting. Its too cold to paint now so I didn't open the blisters to touch it up but I am curious if there are any ideas as to the cause. I just hope they were a couple of somehow contaminated spots.
 
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Hi, what is the substrate please, I.e. brick, sand and cement render, lime etc. what temperature approximately when painted, and what final product was used, incl. primer? These parameters will depend on what the cause will be. Thanks
 
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House is sided with cedar shingles. Primer was Bins for exterior. Paint is Behr supposedly best quality. Painted when it was in the 60's, dry for several days before and after
 
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Thanks for the prompt reply with that information. I'm pretty certain I know what your problem is now. You have bought the best exterior paint for cedar shingles and the primer you have used is shellac based, and is non breathable as a result. Roofers who build roofs from cedar shingle cladding contend this material will last around thirty years if looked after properly. Due to the fact this material was last painted in the sixties, and albeit a durable type it is made of wood. I operate a business in the UK restoring and painting architectural joinery for listed buildings from around 100 to 500 years old. Any wooden substrate, whether hard or soft wood should be coated with flexible materials every seven to ten years. The only reason blisters appear in paint is because something is trying to escape from that substrate.
In this particular instance, due to the fact these walls haven't been painted for so long, and I also imagine there is exposed end grain, (which acts like a sponge for moisture), the blisters are moisture content trying to escape. The optimal moisture reading for cedar to be painted without these problems occurring is around 12-15%.
My solution to remedy this problem, provided none of the wood has wet or dry rot, is to purchase a moisture meter initially from a local hardware outlet, digitally measure the moisture content of the wood, especially where the blisters are, & I can almost certainly guarantee the readings will be between 20-45% at the very least.
If this is confirmed, you will need to wait to remedy your problem until the height of summer and then redo the job. When the temperatures are highest the moisture content of the cedar will be the lowest.
Use your orbital sander, firstly with 60 grit as before to virtually strip the previous coatings, then 80 grit, then finally 120 grit for the smoothest substrate to paint for aesthetics. Moisture test the wood after this process and wait until a couple of hot days have passed to further the drying process, and when you confirm a reading of less than 15% or more ideally 12% or lower just paint the entirety of the walls with the Behr paint without any spot priming. I would also (due to the age of the material) recommend three coats instead of two.

I hope my answer wasn't too long winded and wish you all the best.

God bless and have a great weekend. Cheers
 
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Im so sorry, I think I may have got the wrong end of the stick, you were referring to the temperature when you said sixties? If thats the case it may be resinous knot sap escaping under the blisters. This is also quite common. The way to treat this is to apply a heat gun to the knots until all the sap escapes. when dry the escaped sap rubs down extremely easily.
 
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I appreciate the detailed response. Here's a couple more thoughts. The house was last painted in ????? early 2000's or a year or two earlier. The paint, Sears best at the time, was actually in quite good condition. A bit of peeling here and there, but really very minimal. In general, it was still solid to its prior paint but pretty well faded and chalky. The shingles are red cedar which I understand can be an issue but the old paint had nothing like the couple of blisters I saw the other day. Its too bad the original home owner painted the house, some stain would have been nicer and easier to maintain. Another consideration is the house is tight. Having lived here for 40 years, over time I have redone everything, studs out and there is a solid vapor barrier which when not present, I have seen cause paint problems. Maybe next nice day, I'll go out and pop one of the blisters to see what is lifting: first coat, second, primer? Or what? If that's all the problems it is no issue. I am just a bit concerned that those 2 might be predictions for more. Oh...the 60's was the temperature and that's when the house was originally built. Interestingly, my town lost all its building records about 45 years ago in a fire so details on a lot of stuff is absent. But the best info I can find is the house was built in 1966 so I assume it was then it got its first paint. I would imagine shingling with red cedar today would cause a need for a new mortgage. I don't even know how available they are in my area. Crappy white cedar bundles are going for 35-40 dollars. They look like stuff I would use for shims.
 

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