I have been trying to keep the countersunk heads of my stair tread attachment screws concealed since they were built last summer and have not had much luck. The treads are PT 2x10s with good outdoor screws. I used indoor/outdoor spackling the first time and wood putty (powder) since then. Between the weather and having to put salt on them in the winter nothing I have tried has worked for very long. If this keeps up the screw heads will rust away and I will have to rescrew the treads down again. The treads and porch floor are painted with porch and floor paint. There are 9 treads with 9 screws in each so it is a lot of work to have to keep redoing.