Exterior Priming and Back-Priming

      Woodworkers debate whether shellac should be used for exterior priming. Manufacturers don't recommend it, but anecdotal experience varies. June 17, 2012

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:

From contributor D:
Primer is a little better than nothing; water based primer will warp a 1 x 12 board unless it's vertical grain. Oil based paint is a better vapor barrier too.

From contributor A:
I disagree with contributor D. The trim should be pre-primed on all 6 sides before install. Modern waterborne/water cleanup acrylic primers and topcoats outperform oil based house paints in all respects.

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.

From contributor D:
I think my point was primer alone isn't enough. We pre-prime hundreds of windows a day - and mark my words - a 1 x 12 pine board will warp after priming. Wide casing we do is all engineered.

From contributor B:
Zinser's Bin is actually not recommended for full coat exterior priming. The can states it is for spot priming only on exterior applications.

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.

From contributor S:
I agree with Contributor B. Bin is not a good cover-all primer. A job we are at had some new t&g fir porch flooring installed last fall that was primed with Bin prior to installation. The floor was painted after the install and over the course of winter a good portion of the finish coat peeled. If you are using a water-based primer I would damp wipe the boards before priming to help develop a better bond with the fibers. If oil priming, a thinned 1st coat and full strength 2nd coat would provide better protection and this would especially apply at end grain.

From contributor A:
In as much as I respect your experience and opinion, I have to disagree wholeheartedly.

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.

From contributor B:
As always it seems there are no 100% certainties in any process. I've never heard of shellac being used in boatbuilding so there is another unexpected use of something to learn about.

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.

From contributor A:
Your theory sounds reasonable. I'll see if I can find the issue of Wooden Boat mag. I guess it begs the question... How big of a spot will it cover without failure? I'm guessing about a foot in diameter, which would certainly cover back priming a post. Why did you use BIN if you knew it wasn't rated for full exterior coverage?

From contributor B:
That project was over 25 years ago. It's when I learned BIN isn't intended for that use. I should have done a better job reading the directions.

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