The gap is 1-8mm wide. The window sill is PVC thus probably only selected glues will work. Wall is drywall. This is a low window sill on a high window - so in the right lower corner you see the covered room floor. The sill is just 15cm above floor level.

Edit: talking about gap on the right (Yellow is membrane not yet cut to fit)

Options I was thinking about:

  • fill up with silicone or acryl and level it to the window sill (2K epoxy will not work as the sill is hallow, then I would need to fill up the whole sill, 2K epoxy putty would work)
  • make a silicone joint with a falling angle from the wall (better than leveled to the sill?)
  • glue some flexible profile - would still get a bent line which would propably catch the eye
  • tape it with a durable white tape - (probably not)

(leveling the wall is not an option - too much effort)

Maybe there is some creative idea out there. Else I tend towards the silicone fill + leveling to the sill.

enter image description here

  • Are you talking about the gap on the right, or the black/yellow at the back left? If it's the one at the back, which isn't straight, the window or the sill? If it's the sill why don't you take it back out & fill underneath until it does fit? If it has room to move, then whatever you fill the gap with, chances are first couple of times someone puts weight on it, it will split.
    – Tetsujin
    Nov 23, 2021 at 14:33
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    @tetsujin - talking about the right side - thanks, edited my question. Back shows the overlength of the humidity membrane which just needs to be cut.
    – AndyZ
    Nov 23, 2021 at 16:33

1 Answer 1


I think whatever you use, you'll see it every time you look at it…
… but I bet you'll stop looking at it after a week & not spot it again until next time you decorate.

I'd fill it with 'whatever I have already in the gun, or opened & not yet dried up'.
Decorator's caulk, acrylic or even silicone. Caulk & acrylic paint easier & aren't as shiny.
From comments: Don't try to paint silicone, it doesn't go well, short- or long-term.

First, I'd mask off the lower wall flush to the end of the sill - anything that squeezes itself into that pattern won't be easy to get out & will drive you nuts every time you spot it for the next ten years.
Then I'd just run a bead down along the wall, below flush, let it dry so it will hold a second layer without it all falling through; then run the second layer & level it with the square end of a steel rule rather than a wet finger. That way it will nearly hide itself as part of the sill rather than be a varying width curve.

  • indeed, and if using acrylic caulk, OP can paint to match.
    – P2000
    Nov 23, 2021 at 17:17
  • for sure, though I did already mention that ;)) In the UK, acrylic is a silicone alternative, with caulk - though also made of acrylic - not being quite the same thing. In the end, both take paint easily, whatever their original design concept. [Way too many joint-filler options imho just to get you to buy one for each purpose rather than just use whatever's in the gun ;)
    – Tetsujin
    Nov 23, 2021 at 17:20
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    sorry yes you did. Now reading it again, I could perhaps emphasize that silicone cannot be painted for a durable finish. I have tried and failed at that (precisely because it was too shiny... to your point!). I don't intend to split hairs, but if OP even considers painting, silicone would be out for me.
    – P2000
    Nov 23, 2021 at 17:27
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    Thanks a lot. Well, I had only white silicone at home, needs to be used anyway. Yes, painting will not work, yes, it is shiny, and yes I did a test on one sill (have 5 of them with two sides each) and got a nice thin layer of silicone on the wallpaper as well. OK, for the next will I will mask as you suggested :) Also will need some angled tool to not level silicone completely to the sill, because the edge of the sill is not perfect thus leveling to the sill displays those imperfections at the edge. Thanks!!
    – AndyZ
    Nov 24, 2021 at 12:51

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