Selco mortar plasticiser top coat crumbly and scratches off

[benjiman]

Hi all,

I did some internal rendering and have got a problem with the topcoat.

I did a scratch coat, 4 to 1 mix of plastering sand and cement with 3 in 1 admix (from toolstation)

Waited 2 days and then put on the top coat. (5 to 1 mix) Used Selco mortar plasticiser as per manufacturers instructions. Added to gauging water and made mix. Did not overmix, mis wasn't sloppy, went on fine on the wall.

It's cured not, been a few weeks. Feels solid enough, but I am able to scratch it off the wall with my finger. Doing it with the back of a hammer and it all crumbles off.

I've taken off to the scratch coat and this is solid, cannot scratch this off.

Where do you think I've gone wrong? Could it be the selco mortar plasticiser? I put in the minimum amount as it was only a small area. Plastering sand and cement are the same as the scratch coat which as I say has dried solid.

Any ideas?

 

[roughcaster]

Probably dried out too quick. Did you wet the scratch-coat before you put on the top coat? What was the ad-mix,, did it contain a waterproofer too? A scratch coat, even if you give it a wet down, can still have a fair bit of suction. Anytime I render, i prefer to use a "combined waterproofer/plasticiser" in the scratch coat.
It kills any suction for the top coat render,,
It gives you more time to float up/finish the render.
It also slows down the drying/cure time, giving you a more solid finish, rather than a soft powdery one.
Cement render, plaster, concrete etc are always better drying/curing "slowly" if possible.

You're right about following manufacturers instructions with "any" admix in cement, plus, always measure your sand and cement using a bucket, rather than just guessing by using a shovel. A shovel of wet/overdamp sand, has far more sand on it than a shovel of drier sand, so it's very easy to make a batch of render that's "too sandy", therefore, powdery and weak. Just loosely fill up each bucket with sand, don't pack it/compress it, keep count, same with the cement, then put each one in the mixer.

 

[benjiman]

Thanks Roughcaster. The admix used in the scratch coat was a waterproofer and plastisiser combined.

In terms of adhering to the scratch coat, the top coat seemed to have bonded. When tapping on the top coat it didnt sound blown and when scraping off, chunks of it didnt come off either. So I don't think it has dried out too quickly. There werent any cracks in the top coat either.

The weather was quite mild when I did the rendering, probably 15 degrees. When I applied the top coat I didnt wet down the scratch coat as
it had waterproofer.

I have now taken all of the top coat off with the back of a hammer, it simply scraped/crumbled off. Looking at some of the pieces that came off
I can see loads of air bubbles. Like an aero chocolate bar. Would this be causing the weakness?

The instructions for the plastisiser said 250-500ml at a rate of 20 parts water to 1 part plastisiser.

So in a 10 litre bucket of water I mixed in 100ml. This was less that the recommended amount. So I'm assuming this should not have messed with the mix, and I did not overmix. I used a cement mixer. Mixed only until the desired consistancy, slowing adding water until it was right.

From the aero like structure of the pieces it looks like there is too much air in the mix? But I can't work out why that would be the case, so there must be some other reason?

Yes I did uses the shovel method instead of the bucket. Thanks will keep that in mind for next time. Sand was bagged and bone dry when I rendered this time, but make sense to do it that way for accuracy.

 

[Micilin]

Excellent advice as ever from RC.


Also, unless you know, hard not to leave it in the mixer too long, this will kill your mix so it doesnt set properly. You can tell the mix is gone this way as it becomes runny and looks like it has lots of bubbles under the surface. Hard to describe but easy to spot if you know what to look for. This overmixing is a common mistake and from your description of the render sounds like it.

Your admix was one product, not two? Can you post exact name of it?
100ml seems a lot for 10 l??


Also don't retemper (remix) in the mixer, bits tha tyou have dropped or that have started to set (on your board, ground , barrow, in the mixer ). If you have to , then add cement.

 

[benjiman]

Thanks Micilin,

The admix was Everbuild Evermix 3 in 1. This I used for the scratch coat. The scratch coat is solid, no problems, can't scrape that off at all.

For the top coat I used sealocrete sealomix from Selco

http://www.selcobw.com/sealocrete-sealomix-5-ltr.

I mixed it the same way as I mixed the scratch coat, for long enough until it looked the right consistency. Did not add old mix to the mix either.

hmmm looks like I've used too much? I went by the manufacturers instructions, bit of a bummer if I should have used less!

I used the Evermix 3 in 1 admix for the scratch coat. I had good results with this. For internal rendering, is it OK to use this in the top coat too?

 

[Micilin]

For 10 litres you should have used about 50ml I would have thought . When you rubbed it up with a float , did it get very sandy on the surface or drag a bit?

And for every ten ml more you use, you reduce the mixing time before a danger of overmixing.

For workability (not waterproofing) you are always better to go a bit less rather than more. It may be a little less workable but it will rub up better and you will have less danger of over-entraining. (More than 12% entrainment reduces render/mortar strength but don't ask me how you measure this!)

The trouble is, different sands need more or less. Some are more 'greedy' than others. The trouble is that mixing render is a bit of an inexact science, as RC will probably agree!

If the admix was okay in the bottom coat, you could use it in the top coat.

Personally I use lime in the top coact if I can without any admix, but I don't see the point of you buying more stuff if you can use what you have.

All that said, there are thousands of houses rendered with nothing more than a squeeze of washing up liquid in the mix

 

[benjiman]

Thanks mate...

Nope didn't drag, wasn't sandy...I guess I just used too much! (despite following the instructions!!)

Oh well, next time, if in doubt will use half the amount stated m=by manufacturer. Can deal with working with a less workable mix, but having to take it off afterwards is a bugger!

 

[Micilin]

If there is a big area, you can adjust the mix as you go, maybe put a little more in the second mix if you think it is too 'dead' - but remember, it's not meant to be like a creamy plaster mix, you will need a bit of muscle too.

I think you have it sorted. If you have a hot day , if you have wet down the all the night before as well it helps. And the morning after, a gentle mist on it will help with curing