As soon as possible, flush the stain with cold water. It is best to hold it directly under a running faucet and let the force of the water do its work. Using white vinegar, sponge the stain and then rinse well. Repeat until no more of the stain is coming out.
Treat with a prewash stain remover and then launder as usual. Check for the stain before drying the garment. If it remains, mix a solution of oxygen-based bleach and tepid water and submerge the garment. Allow it to soak overnight and then launder as usual.
If the garment is dry clean only, point out and identify the stain to your professional cleaner. If you are using a home dry cleaning kit, be sure to treat the stain with the provided stain remover before putting the garment in the dryer bag.
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