Installed fiberglass front door then worked hard to stain it so it looked like wood. Three months passed and I didn’t get around to sealing it (ignorance). Looked great until mold started to grow which I can’t remove with bleach or sandpaper. I need to take the whole surface down

I am going to paint (white/semi gloss) the interior of my Andersen picture window. Presently it is stained oak color & polyurethaned (sic) and furniture polish applied routinely for the past 25 years. How should I prepare the surface prior to painting?