I've just done a repair to my concrete slab flat roof over an outhouse, which had become spalled and had disintegrated at the corner - I made some shuttering around the corner and filled with cement; it worked very well. Howvever, I now want to finish the job with a protective layer of bitumen paint over the whole roof, having pressure-washed away all remnants of the previous layer during preparation for the repair.
So - how long do I need to wait before painting? The slab is about 4" thick, although most of the repair (other than the corner) is only about
1" to 1.5" deep. Given that the underside of the slab is exposed to the air, will it be OK just to wait a week or so, until the surface has gone "pale", and assume that the rest of the drying out will happen in a downwards direction? (ie will the moisture pass down through decades-old concrete?)
Supplementary question... on the underside of the roof you can see the some of the exposed bottom of the steel reinforcement to the concrete, which had obviously been far too close to the face of the concrete when it was poured. These areas have become rusted and need some form of treatment. Would a bit of Hammerite sloshed on do the job? Or would I need to cut the surrounding concrete back to expose clean unrusted steel, and do something else? (IIRC most proprietary rust treatments attack concrete like crazy, don't they??)
Cheers David