i had the same problem on my front stoop. i couldnt get the paint to stick. i ended up trimming a giant branch off the front tree, to get some sun on that bad boy to dry it out. it was a really shady area. i "etched" the stoop with muriatic acid ( i think i spelled that right) let it dry for a few sunny days. i bought some concrete primer sealer and then when that was dry painted over it. it seems to be holding... we'll see after this winter tho.