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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Painting, Decorating & Tiling Questions >> Painting concrete https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1242714135 Message started by kamog on May 19^th, 2009, 7:22am

Title: Painting concrete
Post by kamog on May 19^th, 2009, 7:22am

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.

Title: Re: Painting concrete
Post by ymmah on May 20^th, 2009, 4:42pm

Yes, if you don't use alkali resisting primer before using an oil paint the paint will saponify, which means it will go soft and soapy.

Don't suppose there is a fixed amount of time before painting, but definitely wait enough time for the moisture to dry out.

Title: Re: Painting concrete
Post by yoggo on Jun 1^st, 2009, 7:37pm

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