Replies

1. 
   DanH | Jul 18, 2013 08:21pm | #1

   It's perfectly normal for pre-primed.  The rest of the "factory primer" will peel off in about 5 years.

   In the interim you can scrape the spot clean, wash it with mineral spirits, and apply a good alkyd primer.

2. 
   DanH | Jul 18, 2013 08:59pm | #2

   It's really impossible to "factory prime" wood, since good primer should not be left unpainted for any period of time.  And the "factory primer" does not stick well to the wood, in part, I suppose, because the plants use cheap water-based primers that will dry fast and not be sticky, so that the planks can be primed and stacked rapidly.  And the primer is cheap.

3. 
   Hokuto | Jul 19, 2013 12:06am | #3

   One reason the factory primer doesn't last is because the wood probable wasn't properly dried in the first place. Many knots like those in the photo extend through the wood to the back, creating a highway for moisture as the wood dries and shrinks over the years. I used 2x6 WRC for the window trim in my recent renovation, and even without using a moisture meter I figured it was not fully dry, so I stickered it and tried to allow it to dry on-site for as long as possible; whenever I got out a piece to use, I began by filling every tiny crack and knot gap (all six sides) with an oil-based wood putty, then priming thoroughly (x6). The pieces used on the S and W sides of the house got 2 or three coats of primer, followed by an oil-based single-component urethane exterior paint.

    The oldest trim on the west side where it gets the heaviest sun exposure dates from August, 2010, so it's currently about three years since final painting, and at this point the paint job shows no cracks or faults (yeah, I know, knock on wood).

1. 
   bluegoat | Jul 19, 2013 01:39am | #4

   WRC = western red cedar and you painted it?  Here I thought I was mr overkill.

   Sounds like a pretty through job though and if your job doesn't last how can anyone else hope to have a chance.  I guess economically waiting 5 years after it has been installed and then powerwashing the peeling areas, sanding and then re-priming and painting probably is cheaper than paying to fill all then alky prime all the knots ahead of time.

   Thanks for the reply !  I'll bump this in a few years and see how your paint job is =)

2. 
   DanH | Jul 19, 2013 06:48am | #5

   The main reason for the peeling paint over the knots (vs just a crack line outlining the knot as it shrinks) is that the knots contain a lot of resin.  Drying for an extended period will help stabilize the resin (though not actually remove it), and you can wash with mineral spirits (or maybe there's a better solvent) and then use a primer that "bonds" to the resin vs floating on top of it to get a more solid bond.

   But this is one reason why cedar is more often stained.

4. 
   rdesigns | Jul 19, 2013 09:23am | #6

   As DanH notes, it's the resin in knots that prevents paint from sticking and even if it doesn't cause the paint to peel, it will bleed thru and show dark spots.

   The best sealer is shellac, either pigmented or not. Just be sure to buy shellac that is meant to be painted over (unwaxed shellac).

   Zinsser makes the type you need. Or, you can use any shellac sanding-sealer, because they are unwaxed as well.

1. 
   bluegoat | Jul 19, 2013 05:15pm | #7

   Thanks, I'll follow your advice.  I only really care enough about the eye level stuff to go to the effort.  I'll just pay someone to do the rest when it looks awful enough from street level.

1. 
   IdahoDon | Jul 23, 2013 01:14am | #8

   Your knots are dry as they are going to get - simply scrape it and prime with a decent primer.  Kilz2 is easy to use and sticks well to most anything.  Don't spend 2 hours on a touchup that should only take 15 minutes.