Blocking is the undesirable sticking together of two painted surfaces when pressed together (e.g. a door sticking to the jamb).

Possible cause
Blocking happens when a sufficient drying time has not been allowed before closing doors or windows and is more common when using low-quality semi-gloss or gloss paints.
Solution
Use a premium semi-gloss or gloss acrylic enamel paint like Dulux Aquanamel®. Low quality semi gloss and gloss water-based paints can have poor block resistance, especially in warm, damp conditions. Always follow paint label instructions and dry times. Acrylic enamel paints have a better early block resistance than vinyl latex paints.
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Have you noticed small droplets appearing on your fresh coat of acrylic paint? This is known as 'surfactant leaching'. Don't panic - it's a normal part of the curing process. For each problem you’ll find a guide to identifying it, its causes and solutions.
Yellowing
Yellowing is the development of a yellow cast in aging solvent-based enamels. It’s most noticeable in the dried films of white paints or clear varnishes. For each problem you’ll find a guide to identifying it, its causes and solutions.
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Blocking