In my staircase in a 3 inch wide board leading up and away from the newel post, there was a 4 inch crack maybe three times wider than what I would term a hairline crack. There is no strain on this wood, only normal expansion and contraction. This is essentially molding, but would take a lot of work to replace. The wood is painted white, I am repainting. When I examined the crack, I found it had been filled with calk. Removing the calk left a crack about 1/16 inch wide that appears to extend through the board, but is only visible to a depth of about 1/8 inch. The crack has been evident for years so the calk fix hasn't been 100% effective. In similar situations, I have used plastic wood and found it to last at least 6 years without the crack reappearing. I am hoping this painting will be the last I ever need to do here. I would appreciate any comments on the situation and any advice regarding the repair before painting.