Painting old pressboard siding?
What Lam Sanders said, it might not be a good idea to pressure wash pressboard anyway. That will get rid of the paint flakes yes, but board is made out of flakes of wood that might rip off. I don’t know if this is your main residence or an outbuilding so I am not sure how nice you want this paint job to look. You could try using a stiff broom to take off the loose paint and see if that does enough of a good job in the patch that is the worst. Latex is too weak to act like an armor for the siding. You need something that drys a tough hard coat.
