Patching existing render

[Mr M]

The outside of my house is rendered and painted. There were a couple of panels of render which were addled and I have now removed these to patch render them. Can anyone tell me:-
1) Does the surface of the brickwork need preparing in any way, before applying the render ie waterproofing, uni-bonding or something else
2) What mix should I use for the render and in how many coats should it be applied
3) Is the 1st coat a different (stronger or weaker) mix than the second
4) What type of sand is recommended for external rendering
5) Do I need to add a waterproofer into the mix and
6) What are the tricks for getting a nice smooth finish and how do you avoid shrinkage cracking between the new and the existing render when it dries

When I've finished the rendering, I intend to repaint all the elevations. I have posted my questions concerning this in the Decorating Forum.

Thanks in advance for any responses

 

[marksson]

1. Just damp down (tap water) the existing brick/blockwork prior to 1st coat.
2. Depends on how thick the existing render is, say 18mm (10mm 1st coat, 8mm 2nd coat).
3. I use a 4:1 and 6:1:1 on concrete block (lots of different variables for stone, brick, age of building etc).
4. fine sharp (but that depends on where you live (ask your local merchant).
5. Waterproofer for final coat (open debate on waterproofer for 1st coat).
6. To avoid shrinkage add as little water as possible to mix and use lime in the mix. As for tricks to getting a good finish maybe consider employing a plasterer (not being funny but the knack comes with experience). I guess my best tip would be not to play with it overmuch.

 

[Mr M]

Thanks marksson,

So say on day 1, I 1st coat and leave it, after scratching the surface to provide a key and then come back the following day to 2nd coat it, do I need to do anything to the surface of the 1st coat of render (which will now be drying-out) ie damp it down or 50/50 mix of Unibond/water etc. to ensure a good bond.

I also thought I had read somewhere or been told by someone that either the 1st coat should be a weaker mix than the 2nd coat or vice versa I can't remember which way around this was Ever heard of this

Would normal brown builders sand be ok, available from the likes of B&Q or Wickes in small sealed bags, instead of fine sharp sand?

Is this ok to use in the mix for waterproofing http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=102009&ts=53573&id=47829 or would you recommend something else?

I've also got a bellcast to form over one of the windows. As it is obviously going to be thickening out towards the bottom, would I be better doing this in 2 seperate coats over two days or say 1st coating in the morning and then 2nd coating and forming the splayed bit later on in the afternoon, when the 1st coat has started to go off but is still damp

I assume that 6:1:1 is sand:cement:lime ? so the same volume of lime and cement. Is it ok to use both lime and waterproofer in the same mix for the 2nd coat.

Do I need a plasticizer or does the lime keep the mix pliable for a bit?

Take on board your suggestion to get a Plasterer but as this is not a massive area, I want to have a go myself initially and see how I get on. Don't discount me posting back for your mobile number though, if it all goes pear shaped Just on achieving a smooth finish, Is there a specific type of trowel for this or can I achieve this with the same steel trowel used to apply the render, once its started to go off?

Sorry there are so many questions but I want to have a go at this myself and want to make sure it's done right and hopefully the post may assist other forum members in the future