Render: Purpose of 2nd coat?

[Richard Knight]

I continually read about the scratch coat and then the 2nd top coat in relation to rendering, but I'm puzzled as to what the 2nd coat is for? Logically I'm assuming it's either to get the necessary thickness (my next question...) or to get a nice smooth finish? Any ideas?

Secondly, I need to calculate the quantities of sand & cement I need but have no idea on the thickness I need in the render. What is the approximate thickness per coat (i.e. scratch coat and top coat) I should be looking at? I'm assuming then I just work it out in cubic metres.

The plan is to paint much of the wall after it's been rendered, and for a smaller portion of it to apply a coloured Tyrolean finish.

Thanks.

 

[trowelmonkey1]

Hi Richard,

I'm no render expert but what i understand is that:

by applying 2 coats, putting waterproofer in the scratch coat will give you more time to work with the final coat making it easier to get nice and flat, also been told that if you do it in one coat, brick/block work can show through.

2 coats should also help with cracking, as you would differ the mix ratios for each coat, i think making it stronger for the float coat so more sand, best to get a render pro's advice on this, can't remember if it's the scratch or render that needs to be strongest, a lot of folk use lime in the mix too, makes it more flexible AND breathable.

regarding quantities try this: http://www.source4me.co.uk/calculate_building_materials.php

 

[Alastairreid]

"from the archives"........ courtesy of Roughcaster


Use plastering sand to cement render. 4 sand-1 of cement (4-1) for scratch coat with a combined waterproofer/plasticiser added. If the blockwork has a good enough key, you wont need to use PVA on the wall or in the mix. Just give the blockwork a wet down as you go, and apply the first coat to the wall say 6 or 7 mm thick, and then scratch it, ( an old 6" paint brush handle with "small" nails hammered in along the top works fine), scratch just deep enough to provide a key, but not right through to the blocks. There is no set thickness for the overall render, but the two coats combined would be around 15mm give or take the odd ml. The top coat, applied a day or two after the scratch coat has set and cured, would be a mix of 5 plastering sand, 1 of cement and 1 of lime (5-1-1). A waterproofer/plasticiser would be added to the water prior to mixing. Again, wet the wall before you top coat it.To "float off" using a wooden float, is to close up the render surface after being screeded/straightened off with the straight edge or darby. Floating is a process that is ongoing, and involves going over the same area of wall with the float a few times as the render goes through the stages of going off. When the render is at the final stage of "floating off", it is then sponged over (sponge finish)to give it more closer finish, and an even, slightly sanded texture.. There is a lot of work involved in cement rendering and if you're without experience, you have a daunting task ahead of you, but good luck to you anyway. A good dry day with not too much sun is perfect, you don' want it to dry out too quickly.

 

[roughcaster]

On the area of rendering you're planning to use tyrolean as a finish, "don't" use waterproofer in the top coat render mix,,, just a plasticiser. You need some suction in the render, for the tyrolean finish to build up.

Roughcaster.