Before staining a new woodworking project or refinishing a piece of furniture, you want to make sure you can detect any defects, such as sanding swirl marks or scratches.
In a cup, add a few ounces of mineral spirits and then pour in several drops of the wood stain.
Dip a clean cloth in the stain and wipe it onto the wood. Any defects will become clearly visible.
Let the pre-stain dry for a few minutes, then sand it again, paying particular attention to the area where you saw the flaws.