We cleaned a couple of fibre cement slate roofs lately and the SH didn't  work very well. The weather was pretty cold the first day but it was pretty mild the second day. On both of the roofs a white residue was cleary visible when the slate dried. I think that it came from the underside of the slate which isn't painted. I think that the mix was too strong and ate into the fibres. What would be the best way to enhance the mix while reducing  the amount of SH?

Would a mix made with hot water work better as well?

Michael