You can paint directly on the Headsculpts without any underlying coat. Some prefer to apply one for good measure (and their own good reasons), but it isn't strictly necessary. The only prep work I do recommend is to thoroughly cleanse the sculpt in soapy water before painting.
When repainting the sculpt, use something to hold it to keep your fingers from ruining your work. I use pens or markers depending on which one best fits the neck hole.
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