Bloom on wet exterior varnish

We've had a new porch & door fitted recently but we noticed the other day that the varnish on the sill beneath the door develops a white "bloom" when it gets wet.

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What's causing this? Is it a problem?

Tim

Reply to
Tim+
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This is rather speculative, because I don't really know, but IME it happens on new varnish that hasn't yet fully polymerised and hardened, and is still a bit 'green', even though superficially 'dry'. It seems that the varnish is still capable of absorbing water and forming a sort of emulsion within the varnish layer, resulting in the whitish spots seen in your photo. It's obviously reversible, as the effect disappears when the water eventually dries. The effect seems to become less over time (weeks/months), I presume as the varnish finally hardens completely. I doubt that it's a problem.

Reply to
Chris Hogg

I would tend to concur... I presume it was a water borne finish? It ought to cure fully, but may have difficulties if it was applied in the cold and the damp.

Reply to
John Rumm

Thanks for the feedback. This concurs with info from the joiners and their varnish suppliers. I shall stop worrying.

Tim

Reply to
Tim+

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