New answers tagged sealing
The solution might be a Stripper cleaner. Tile Doctor has a tile and stone cleaner, any acid will damage your stone.
There really is no repair once hardboard starts to decay. You could paint it to prevent some future moisture intrusion, but it won't make the siding more solid. A good quality latex paint will add a bit of life to it. Whether that's worth the cost and effort is debatable. I'd replace any seriously rotten boards and ignore the rest until you re-side.
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