I had an approx 40-50mm cement screed laid on my concrete drive to level out a lot of dips and hollows. The guy who did it used a 4:1 mix, pva'd the old surface first, added a waterproofing admix and used soft sand. This was done over two weeks ago now and is still not right. everytime it rains it washes up a whole load of sand and every time its dry the surface justy dusts up. even where it looks perfectly ok you can scuff it with your shoe and take off the surface. It wasnt subject to frost and wasnt laid in hot weather, conditions i would have said were pretty good. Is it too late to do anything? Any ideas? TIA, and hello!