Question
I'm in the process of wrapping beams and columns for a covered deck with clear pine. 1X12, 1X10, 1X8. Is there a benefit to back priming? I am in Pittsburgh, so humidity variation is prevalent.
Forum Responses
(Architectural Woodworking Forum)
From contributor O:
Yes.
You can use BIN shellac primer as an exterior primer as long as it's topcoated (which all primers should be). It is great for sealing the endgrain of clapboards and trim as you are installing them. Dries in 15 minutes and is the best moisture sealant bar none. We use it for back priming posts as well.
I have firsthand experience with this issue. I had to paint a small building once that had knotty pine siding. We used Bin as a full coat primer and then used extra coats on the knots. After 4 or 5 years the knots were still not visible but the entire building was peeling. This was because shellac does not move or stand up to moisture very well.
Stay with the oil or acrylic primers. Personally I like the acrylic Seal Grip by Pittsburgh Paints.
I built 12 new flower boxes for my own house out of D & better WRC (which is full of knots). I brushed on 2 quick coats of BIN inside and out. My wife was supposed to topcoat them. Fortunately for the sake of the experiment, she neglected to do anything other than fill them full of dirt and flowers. There they sat in Connecticut full sun/rain/snow for 2 years with only BIN inside and out. I inspected them. None of them had any visible damage whatsoever. It looked like I had primed them a month earlier. Then we topcoated with Muralo Endure when we painted the whole house.
Beat that boys!
In Mystic, CT we have some of the best wooden boat builders in the US if not the world. It is common for them to use shellac flakes mixed into various viscosities for boat bottoms. They often make it to seal the wood before placing fasteners. It is used in place of "red lead" which is used by some builders to accomplish the same thing. There was a wonderful article in Wooden Boat magazine specifically about the use of shellac in wooden boat construction.
I cannot comment upon your small building. You need to provide more info on prep, wood, interior conditions, etc.
We have been using BIN for exterior uses such as I mentioned for 20 years, with no problems. In that time frame I have seen and personally created numerous exterior painting problems with both oil and wb primers. If you sand a piece of wood beyond 100 grit, exterior oil paint will often not adhere at all. I lost about $1000 learning that mistake.
Please explain to me in some logical fashion why a primer would work in a spot situation and not for larger areas.
The products specs for BIN state that it is for spot coverage in exterior use. I think you should email them with your experience and knowledge and see what they have to say. Should be quite interesting.
As to why spot and not full coverage priming on exterior applications, I always took it to be due to wood movement and moisture content. The pine clapboards we primed with BIN could have had a high moisture content and also probably moved a fair amount with seasonal changes (flat sawn vs. quarter sawn). The larger continuous BIN coating of full coverage did not expand and contract with the wood, hence resulting in cracking and peeling. The very small patches of BIN in spot priming don't experience the shearing effect of the wood moving below the primer to such an extent. At least that's my theory.
