I need to gloss paint concrete and cement render. Is it true I have to use alkali resisteant primer? And how long do I wait before painting new concrete/cement render?

Your advice will be appreciated.

I'd like to add my experience with painting a new concrete window sill about 2 years ago and ask a question.

I waited several months before painting, then used Dulux Trade Alkali resistant primer, when dry used Dulux Weathershield u/c and then Dulux Weathershield t/c. The top and undercoat have now cracked badly in places. I scraped all the loose off (it lifted very easily indeed in 'sheets' a few inches long by width of the flat scraper and I can see the original primer - looks was quite sound, uncracked, not dusty and quite dry. The underside of the scrapings had no trace of primer - looks just as if the u/c failed to stick to the primer. Anyone any ideas why or what I should do next?

The weather being hot and sunny I intended to wait until the sun had moved for the sill to be in shadow before deciding whether to scape more of the seemingly firm paint. By that time, about 2 hrs later, more t/c and u/c had lifted at the edges. The primer really does appear to be sound and there is not trace of it on the underside of the scrapings