Did a synthetic slate roof today and after rinsing there was some white residue left on the slate, (sorry don't have any pictures). I can only think that it is from the underside of the slate which isn't coated with the paint. Is this caused by the mix being too strong and causing the fibres to dissolve. Again today the algae didn't dissolve as well as it did when the weather was warmer even though the weather was very mild. Would the accelerator help prevent this sort of problem while firing up the mix?

Michael