Rendering question

[RogerS]

We're having the front of our house re-rendered. The wall was prepped, cracks filled and a coat of waterproof PVA and sand to give a good key to the scratch coat.







However when he started to put the finish coat of render on, he struggled the first day because the plastering sand was quite fine and he was finding that as he was trowelling off the finish coat was starting to pull away from the scratch coat. The next day he brought some sharp sand along to mix in with the plastering sand and that made a huge improvement.

I was up on the scaffolding today and discovered quite a few 'blown' areas especially where he had the first days' problems - some of them circled in the photo below.



Now my dilemma is this. Do I get him to remove the blown areas and re-re-render. The trouble with this is that I'm concerned that these patches will then stick out like a sore thumb thus defeating the whole object. Or do I keep my fingers crossed that some good coats of paint will keep the moisture out and that the blown areas won't fall off six months down the line.

Appreciate any thoughts.

 

[MartinCox]

My experience is that patches will show and that painting over (and hoping) is going to disappoint you. I think a good tradesman will see the issues and know he has to take off large areas or all the top coat and re-do to get a lasting, quality finish.

 

[RobinBHM]

I think it may be worth posting on plasterers forum. Rendering is not straight forward, there are lots of pros on there that could advise.

 

[blackrodd]

Plastering sand is not soft sand at all, so he is using the wrong sand, It should be SHARP sand, pit sand is best, far superior to sea sand, ( Wickes sell pit sand ) as too much salt gets through the washing process with sea sand.
That old render should be taken off, The only reason to leave old render is because it would damage the brick or stone work to remove it, and Never render over painted masonry, disaster.
Should be lime in the mix too or the lime mortar will blow modern cement on the scratch coat.
This is why re render is so very expensive, not knowing what lurks behind the paint, as money cost is in hacking ALL the old off carting it away, and in the preparation for plastering.
TBH you may not have the right man for the job there.
(I spent 20 years dry lining and plastering, so DO know what I'm talking about)
Regards Rodders

 

[Dusty]

I dont mean to rain on your parade , but ......... PVA should not be used as an exteral bonding agebt , it will only re-emulsify when moisture gets to it again .The same reason you wouldnt use standard PVA on woodwork for outside . Builder PVA is cheap and is not anything special

Redering on top of paint is pointless as there is no mechanical key to the building .It relies on being able to bond to the original brick or blockwork / stonework . Hence the reason you scratch the first coat .

As said by Blackrod , Rendering sand is not soft sand .

At the very least the original top coat should of been hacked off , base coat too if possible , then scratch coat ( sharp sand ) and top coat ( sharp sand ) . We do add a shovel of soft to a gauge of top coat to help get more fines in the mox as some sharp sand lacks it and it makes it a problem too get a smooth finish .

And as said , patching will look aweful , you will always see it and you will get shrinkage . You will probably find it has blown because it was over worked or it was put on too thick and bubbled . In an ideal world , you want to scratch one day and top coat the next , especially at this time of year as if the scratch coat is left to dry too much , the top coat dries too quickly and "blows" or goes powdery .

Sorry to be the bearer of bad news , but I dont think it will last the years you hoped .

Cheers Dusty

 

[Lons]

I'm no expert though as a builder I have done some rendering but looks as if your guy has made a mess of it. As said, plastering sand is not soft, it's sharp sand which has been graded down to specific maximum sized patricles specifically for that purpose, (surplus is great for lawns and the garden btw).

Patching is a poor solution and it needs re doing or you'll never be happy with it.

Bob

 

[RogerBoyle]

I Agree with the last 3 or four posts above
I'm not a builder but I have been involved with them enough to know what's right and what's wrong. And unfortunately I think you also realise that in this case its wrong and it needs to be redone :shock: :evil: