Washing Gunpla parts with soap & water is one way to prepare them for painting as opposed to priming and sanding. According to this video
from Hobbylink Japan (skip ahead a little), washing the parts removes a special coating that eases the removal of runners from their molds but makes them harder to paint. And compared to priming and sanding--where you risk breathing chemicals or dust that can affect your health--soap and water is a much safer method.
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