Rendering - Problems

[sako]

Hi Chaps.I have scratched coated a wall and some brick built pillars with no problems using 4-1 mix but i am experiencing problems putting top coat on.I have only attempted the pillars as i havent done it before.The first attempt failed and was hollow sounding after it had dried but i think that i tried to rub it up too soon as wooden float was succing to surface.My recent attempt has also failed and i dont know why as i soaked the scratch coat first and waited for as long as poss before i had to rub up with sponge as light was going.the top coat was also 4-1 with 250mls of waterproofer (only used 1/2 bag of cement) and mixing guidlines say up to 500mls 0f waterproofer per 25kg of cement.So can anyone help please.Cheers.
sean.

[DNgroundworks]

Try and SBR bonding coat - paint it directly on to the scratch coat leave it to go tacky, then fire away.

Did you use waterproofer in the scratch coat?

How thick are you putting on the top coat?

Also i was taught that 4-1 was to strong a mix for a render, can cause "crazing" we always used to use a 6-1 for the scratch and then a 5-1 for the top.

[sako]

Hi.I am going for a top coat thickness of around 8 to 10mm and the scratch coat was 10mm thick as i used wooden grounds of that thickness to screed off.Both the scratch coat and top coats ae 4-1 and both with waterproofer in.I was thinking of trying pva and recently had a price for the ronacrete sbr which was pricey.The thing is most plasterers had said just soak wall and fire it on but something i am doing is not right but dont know what?
Is getting the correct amount of waterproofer absolutely critical?
sean.

[henpecked]

6-1 provides a bit o give in the base and the 5-1 is a little bit harder for the top but still less rigid than a 4-1.
The SBR is the way to go if yor not getting your mix right. Although, it should adhere ok to the scratched surface of your base coat.

Hp

[DNgroundworks]

No its not realy, it only matters in the scratch coat, because the wall you are rendering will have some amount of suction, alot if its an old dry wall, and the waterproofer in the scratch coat will help eliminate this when you come to put the top on, if it just some brick pillars you are rendering theres no real need for waterproof in the top coat just a good plasticiser, and like i say bond the scratch coat first with sbr.

10mm is fine, i used to put it on with the steel float, and then rub it when it had "tightened up" afterwards with a sponge, never had to go near it with a wood float - pretty flat from the off ha ha!

Sbr is way better than pva, pva is water soluble and can lead to problems were damp conditions are present, sbr will bond in dry, wet damp etc etc.

Hope this helps.

Oh and yes wet it in thoroughly before you bond with sbr.

[DNgroundworks]

And dont mess around with it with the trowel to much, i think that is an important point as it will just lose its shape, sag and either fall off or sound hollow when its set.

Kind regards daniel.

[Mikey_C]

SBR isn't that expensive £16/5L from toolstation and will deliver that to your door next day if you are not near a branch.

[sako]

Hi Thanks for the asvise chaps.I have got it into my head that the reason it is sounding hollow is because i am rubbing it up at the wrong time,could this be the case?And how do you know when to rub it up?It is actually 2walls and 2 brick pillars that needs to be rendered its just i am trying pillars first.If i have to go for a bonding agent i think i will use the ronacrete cement slurry option.

[DNgroundworks]

Well that'll work but so will standard sbr just as well, i cant see how rubbing it up at the wrong time will make it sound hollow, unless maybe you are rubbing it up when its to wet and then its sagging, its more likely that you didnt use enough waterproof in the scratch coat and excessive suction is drying out the top coat too quickly - sbr will help this.

You rub it up when you can touch it with the sponge and you dont move the main body of the render, just leaving a rough but not too rough texture.

[sako]

The sbr from toolstation reccomended by mikey_c in the post above would this be ok?Also with this type do you just paint it on straight from the container or is it diluted first then painted on? Cheers.
sean.

[DNgroundworks]

Yeah that should be fine, in your situation id dilute it 50/50 but in extreme situations id paint it on neat.

[sako]

Thanks for that chaps.A bloke in tp said today that it is important when rendering that the scratch and top coats are of different strength,in my case the scratch was 4-1 so i am hoping i can get away with 4-1 in the top coat by using a sbr or similar product.I tapped the pillar earlier today and the hollow sound is just in the center of the pillar about 4" dia (the pillar is 300mm square by 18" high) it is just a feature pillar and not structural.
sean.

[DNgroundworks]

4-1 is a bit strong IMO id use a 5-1 ratio mix.

[sako]

Do you think thats why i have had bonding issues?I will put top coat on at 5-1.So the other part of the wall i need to do is yet to be scratch coated so what would you reccomend for scratch coat (ratio) basing the top coat on 5-1.cheers
sean.