X-90 is a Masonite corporation product. Hardboard siding is what it is, but the x-90 is the trademark stamp put on the back of the sheets of a Masonite panel.
These have issues with moisture, before you paint contact Masonite to see if your year product has a claim against it. Chances are the money is gone, but then you will know if it should be painted or replaced.
Spend your money wisely.