If paper sizing isn’t applied in the correct amount, your watercolor painting will appear patchy when you apply a wash of color. In addition to that, watercolor paper with incorrect sizing will have splotchy or grease-like marks on the surface.
The good news is that you can correct the paper sizing by immersing it in water and cleaning the paper surface. Because of this, surface sizing is dissolved and dispersed across the surface. Plus, greasy marks are washed away due to soaking and brushing the paper.