Re: Eliminating mold stain.
You can either shoot the area in question with clear lacquer or a BIN primer such as Zinsser red label. Use several coats to get a good seal, then apply two coats of paint.
Whenever you seal a stain like this, it's going to affect the gloss of the top coat. To minimize this, you can prime the whole board, use a deglossing agent after priming and before paint, or extra coats of paint in the area where primer was NOT used.
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