White stains on impregnated wood with resin are usually caused by a chemical reaction between the resin and the wood-impregnating agent. Resin is naturally a sticky, viscous substance produced by the tree to protect it from insects and disease. When wood is impregnated with chemicals such as copper salts, a reaction occurs between the resin and the impregnating solution.
This reaction can result in the formation of white stains on the surface of the wood. These stains are also known as resin bleeding. The extent to which this occurs depends on the amount of resin in the wood.
Although resin bleeding can be unattractive, it does not affect the structural integrity of the wood. However, it can lead to discoloration of the surface which eventually will fade out due to exposure to sunlight.