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Exterior Window Sill Paint Peels Annually

We have lived in our house now for 17 years. I am hard pressed to remember a year when the paint on our south side window sills has not peeled, which is beyond frustrating. We have tried oil based primer with oil base top coat, oil based primer with latex top coat, and last year I applied Thompsons Water Seal, waited three months, then applied oil based primer with a latex top coat. This spring, there was a bubble, a crack, and then shortly after the entire coat of paint came off in long sheets, primer and all. My wife is a very good painter--each time the surface has been sanded, thoroughly cleaned, and then painted. It never peels on the front face of the sill, nor does it peel on the vertical pieces; just the top surface. Right now it is bare as we consider our next plan of action. The side of the house is brick; the window is a double pane vinyl; the sill is pine or hemlock; and it rests on brick that is set sideways perpendicular to the wall at about a 45 degree down angle. The brick is treated with a water repellant sealer and there is a membrane barrier that was applied to the underside of the sill at the point where the sill and the brick contact. Can someone provide ideas on what to look for as the cause? Is the situation possibly too hot with the double pane window reflecting the heat and sun down on the sill baking the paint; or maybe the brick is soaking up moisture even though it is sealed and the sill is absorbing that moisture past the barrier like humidity or something? My wife is suggesting that we replace the brick that is placed at a 45 down angle with stone like granite or something and replace the sill with 5/4 cedar.

Re: Exterior Window Sill Paint Peels Annually

There IS water getting in....SOMEWHERE!

* How old is the home?!
* Vapor-barriers present?
* How's the insulation levels, and what type?
* Most likely...water is coming thru mortar-cracks above the window, and wicking down into the wood from behind...and/OR from uncaulked or poorly caulked areas.
* Rent a moisture-tester so you know how dry this wood area is.

Bottom-line....Water is getting to that wood...SOMEHOW.


Re: Exterior Window Sill Paint Peels Annually

I don't quite get the picture of why a vinyl window is set on a wooden sill? There are plastic dimensional lumber products on the market. Perhaps the sill could be replaced with such a product.

I have never liked the use of brick under windows. Eventually the mortar loosens and leaks. Also, some brick types are extremely porous and wick water. Your wife's idea of replacing the brick with a non-porous stone is not a bad idea, but would be quite costly. I am sure Holmes on Holmes would do it :) Nothing is too expensive for him!

I recall a very expensive home I did years ago where the builder had used porous brick sills. The house had over 60 window openings. We had to replace several of the sills which had rotted or the finger joints had started to separate. I commented at the time that some future homeowner was going to have to suffer the cost of replacing 60windows and sills due to a construction shortcoming. The house was less than ten years old at the time. My customer chose to move rather then be caught having to suffer through the costs and mess of replacing all the windows in the house.

Re: Exterior Window Sill Paint Peels Annually

Check the wood with a moisture meter before painting. If it's too wet, you can't (successfully) get any paint to adhere for long. If necessary, tent the sill so it has a chance to dry out completely, to 10% moisture.

Re: Exterior Window Sill Paint Peels Annually

Thank you all for the comments. The house is 56 years old. The windows were installed about 10 years ago. The sill had to be replaced about 2 years after that because there was a flaw in the window that allowed water to leak behind the caulk line and rot the sill. The previous sill was a nice chunk of cedar. We keep the window and the sill caulk in good shape using polyurethane between the brick and the sill.

The house is well insulated top and bottom; but, I can't say much about the walls.

So it looks like 10% moisture is the limit for moisture content and I will get a moisture meter. Cracks in the mortar could be a possibility as the south side gets weather worn.

Re: Exterior Window Sill Paint Peels Annually

You should change your wooden seals because after a time they got broke and that's why the moisture take a place in mid of the broken seals.

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