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If you've painted the furniture, let it dry for at least 24 hours before you begin. If you're not painting it, wipe the furniture clean with warm, soapy water. Then let it dry completely. Once you've finished decoupaging the furniture, leave it to dry in a well-ventilated space. The furniture should be dry to the touch before you apply a varnish. Follow the manufacturer's instructions about drying time since they'll vary by product.

Summary:
Clean and dry the furniture. Let the furniture dry for at least a 2 to 4 hours.