Re-render

Hi All. I want some advice please. The rendering on My small gable end on my cottage is coming off, there are 2 layers of rendering, the top layer is 70% off and the rest of the top layer is loose so I am going to scrape it off, the base layer which is onto concrete block is good and sound, the trouble seems to have been that to me, the first layer should have been "Scratched" to allow the second layer to grab onto, but the base layer is smooth. I want to re-apply a fresh top layer myself but as I said earlier the base layer is smooth.
Total area is 15 sq metres, is there some kind of bonding agent I can use to get the new top layer to stick to the base layer, and are the bags of ready mixed rendering cement in BnQ any good, on the bags it says 1 bag will do 1 sq metre. The neighbour who is a retired plasterer, says that my best bet would be to remove the remaining loose top layer, then apply thompson weathershield to the base layer as the job in question is on a East facing wall and gets very little rain on it . There is no damp penetration on the wall. thanks for any advice

I wouldn't listen to your neighbour!!If the top coat has "blown" off then It is probably like you say because it had no key. Also I would not attempt to do this job yourself . Unless you know what mix was in the base coat, what ratio of sand to cement you are going to put in the top coat,are you going to add lime? Are you going to "scutch" the existing render before you r-coat it? Are you going to knock off the base coat and start again so you know what strength the scratch coat is so you can put top coat on to suit. Or will you be grinding diamond patterns in the existing base coat and "SBR"ing it before you throw your new top coat on? Are you going to "float " and sponge your new top coat?Does it need a drip bead on your DPC? Have you got a safe scaffold to work off?
If you can answer most of these questions then you are capable of doing the job yourself. It takes years to understand external rendering and coatings and you have to be taught how to and how to understand the substrates and backing coats that may or may not be the right ones for different jobs. We want people to do most of the work themselves this is why it is a DIY site but there are certain things we cant tell people to do with out actually showing them, so if I were you I would get 3 local plasterers in (who have been reccomended) and see what they come up with...

Roy's reply says it all. So much involved, so much to go wrong. Get quotes as he said from people with a good reputation,, and also, make sure you get a good quality plastering sand.
I've never bought sand from B&Q, so i can't comment. I get any sand i need from a local quarry, where it's washed and the colour is constant,, or from our local Travis Perkins.

Thanks for all the replies to this. I got a local plasterer in today to look at the gable end, he was recommended by a good roofer I used a few years ago. He says he will erect scaffolding,scrape off all the loose rendering, leaving any thats firmly attached if any, clean the revealed base layer down then apply bonding agent to the exposed base layer, he says he has had a lot of success with this method, he will do the gable and some more loose top layer we found round the side of the house which is also onto block for £1000 , plus an extra £500 for scaffolding supplied by a local scaffolder. Total about 24 sq metres, does that sound right to you, and how do you find out what mix is in the original base layer, there could be lime in it for all he knows, I dunno, after reading your replies that all sounds dodgy to me. when I told the guy next door who is a retired plasterer, he said "if he does that It will all fall off after 2 winters". and repeated to me to either seal up the base layer with a breathable sealer or paint it as its in good condition. I dunno what to do now, the walls not damp at all but I will myself remove all the loose top layer as its trapping water in areas where its loose.

Like I said you should get 3 plasterers to come and have a look at it. £1500 for 24mtrs sounds a lot to me. As it sounds like he will only be putting a top coat on it. But I dont know what price per metre the lads are charging nowadays, maybe some of the lads on here will tell you. Get a break down of his pricing....

It does seem a tad expensive. Not only for your renderer, but the scaffold aswell. As a rough guide, i just done a similar sized gable, that got taken back to brick for £700 plus £350 for scaffold