Paint cracked over caulk

1 Jun 2016
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United Kingdom
hi my husband has recently has recently papered our little boys room with a thick insulating paper as he was a really cold outside wall. He's caulked the gaps then painted it with a dulux Matt paint. It cracked in places just on the caulking so he gave it another coat and its still cracked. Plus you can see the vertical line where the caulking has been done. It's seems like we need to use an oil based undercoat then repaint. If we do that do we need to paint the whole wall not just the caulked bits so it's all even. And is it just any oil based undercoat we use.
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What are these gaps that he has caulked? Decorator's caulk is normally used between timber skirting, architrave, etc., and plastered walls, before the timber is painted.

It was used to even out the edges of the paper where it joins as the paper is 5mm thick. In some parts it's fine it's just you can see the fine cracks on parts of the main wall.
Not ideal; caulk isn't for that. How soon after the paper was hung was the caulk applied? The paper is likely to have moved as it dried out.

I don't see any point in using oil-based undercoat. If it's all dry I would paint the whole wall again with emulsion and see what happens. No guarantee that it won't continue to crack, but it might be OK.
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It was caulked the next day. It was definitely dry. He knows it's normally for that but the paper was so thick. It did say on the manufacturers instructions so that's what we did. It's had 2 coats now so not sure another coat would make a difference that's why I said about an undercoat or primer.
Sometimes emulsion will crack on caulk, and oil based undercoat usually solves the problem, or you could use a high grip primer if you have any around. Dilute some undercoat with a small amount of white spirit so that it will go on thinner (which should stop the difference in paint films noticing) and then re-paint with the emulsion. You shouldn't have to emulsion the whole wall (and definitely don't need to undercoat the whole wall), but with new paper, you can often see a difference if you just touch up spots here and there.

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