Is mildew covered under the warranty for okoume? It seems to me that 'marine grade' plywood shouldn't get mildew damage. I think that some suppliers (CLC, Pygmy?...) have okoume that's insured by Lloyd's of London. Although you would have to start from square-one, maybe a warranty claim is a path of recourse...

If the mildew doesn't come off or the attempted fix ends up worse than the mildew, remember that it's no shame to have to paint the kayak. Paint it black with a yellow Batman symbol on the side and call it the BatYak. (Put a plexi-glas windshield on it in front of the cockpit coaming;-)

And no, I didn't breathe too many paint fumes!

Dean