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paint question on exterior windowsill

JayTran's picture

paint question on exterior windowsill (post #192581)

I have a problem with a couple of replacement windowsills on the southern side of my house.

They replaced the windowsills when I had vinyl windows installed and the wood is not as stable as the old windowsills were. I am getting a couple of recurring checks that end up splitting the paint and allowing water into the check. Every fall I apply a couple coats of paint and in short order the paint cracks at the check and is susceptible to all the rain we get

here in the NW.

Does anyone have any ideas how to fix this problem, short of replacing the sill?  I've tried exterior spackle to fill the check, that didn't work. Maybe flexible caulk? I think part of the problem is the check is not real wide or deep so I can't get much caulk into the check. Any suggestions would be appreciated.

I have been using oil based primer with good quality latex top coat. I think the wood is fir or hemlock


Jay. No product sold is going (post #192581, reply #1 of 4)

Jay. No product sold is going to do anything but clog up the checks in the wood, maybe even trap moisture inside. This is because all wood expands and contracts on a daily basis with heat, cooling, and especially when taking on water. "exterior spackle" is pretty much a joke for an unstable substrate such as a window sill, which is going to get more abuse from the sun and rain than almost any part of the house. Caulking will fail just as surely. Even if you were to splice in new wood, the movement at the glue joint would ultimately reject the repair.

The good news is, the solution is quite simple. In order to create a substrate that won't move (and thus reject any repair material) you have to stabilize the checked wood. This is done with a liquid "petrificant" or hardener. Minwax makes a good one. use a scraper to remove all the paint around the cracked wood. Make sure the wood is as dry as it can get (don't do this in a rainstorm!) then saturate the area with the hardener. Saturation is the key. I like to use a sharp knife or even a needle to poke holes in the surface above and below the check, to make sure the entire area is well soaked. Use a brush or sponge to keep the hardener from pouring off the edge, and don't stop applying the hardener until it stops "drinking" it in. Wait a few hours (follow the directions) then use a good two part (epoxy) filler. Forget spackle and caulk - you are doing auto body repair now.

Try to get the repair as smooth and flush as possible with your putty blade.  It sets up in minutes and you don't want to be sanding this stuff too much. Don't get any filler on the un-hardened wood, as it will be rejected and take your paint with it .  Wait half an hour, then sand, prime and paint.   

good luck, 


I'll. Be. Back.

Thanks Saul, This approach (post #192581, reply #3 of 4)

Thanks Saul,

This approach makes sense to me. I'll probably do it this way.


At the very least you have to (post #192581, reply #2 of 4)

At the very least you have to strip it down to bare wood, prime with a GOOD primer, and then repaint.  I'm very much partial to alkyd primers,  Moorewhite in particular.

Likely the windowsills were "factory primed"?  I've never seen a factory primer job on wood hold up.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

If you want to fill the (post #192581, reply #4 of 4)

If you want to fill the checks in, use something to clean them out first like a dremel tool with a carbide buhr.  Then use a epoxy filler to fill the cleaned out area.  And use a consolident first.  I'd look at proucts offered by  West Systems