Symptoms
When humidity is trapped in woodstains or varnishes, white spots may appear.
Cause
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Damp substrate or humid atmosphere when coating was applied or during drying
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Porous varnish/stain type
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Water getting in (moisture ingress)
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Insufficient layers applied
Solution
Remove old varnish/stain layers; if necessary treat with wood bleach, which will reintroduce the original wood color. Consequently treat with a new system, for example a varnish like Cetol BB or a good quality coating like Cetol BL Opaque.