Painted masonry prep - to sand or not?

[gav1nnn]

Hello all,

I would be grateful for some considered advice. Is it necessary to sand painted masonry before applying a fresh coat of Dulux Weathershield, in the absence of flaking etc?

I have a painter who insists that 'doing it by the book' involves brushing off any dust with a stiff brush and then applying the first new coat of paint on top of the old paint without any kind of sanding. I find this hard to believe.

Many thanks,

Gav1nnn

 

[TheDec]

Your Decorator is correct, if the painted surface is intact then there is ony a need to remove any accumalated dust by using a stiff bristle brush.

Dec

 

[gav1nnn]

Thanks TheDec.

I had assumed that sanding would be required to provide a key and remove surface impurities etc., and am therefore curious why it is not considered necessary for painting exterior masonry.

Incidentally, Dulux's Site Work Instructions (referring to BS6150:2006) state that 'each coat shall be rubbed down before the next coat is applied' (section SW4.01) in respect of new paint. Why would you rub down new paint, but not old?

Thanks again.

 

[vv2806]

I am not a professional decorator but how are you going to sand textured surfaces, e.g. render, or textured concrete blocks?

 

[gav1nnn]

These are previously painted surfaces (paint on masonry) that are being repainted without being stripped.

 

[vv2806]

My point is - it is not always possible to strip or sand a painted surface. My house is rendered (textured finish) and was already painted. I had it repainted. How was the painter supposed to strip the old paint. Or to sand the render. There are situations when this is simply impossible. Apart of that, brushing with a stiff brush would provide some key, I would imagine.

Did you really read the whole document you posted a link to? You could probably spend that time painting/sanding/stripping your masonry

 

[gav1nnn]

More specifically: ...smooth masonry surfaces... - window cills, bay window columns, porch, that sort of thing.

Thanks for the sarcasm by the way, vv2806, very helpful. I am forking out some good money and want to be sure the job is being done properly. Set against that cost some time on the internet is not much of an investment. And no, I used the index.

I ended up posting here because searching the internet I found nothing whatsoever about the correct procedure for preparing sound previously painted masonry surfaces for a new coat of paint. All technical quidance that I found (e.g. provided by manufacturers) refers only to flaky, chalky etc surfaces.

 

[joe-90]

Which should have told you that is wasn't necessary.

 

[vv2806]

Which should have told you that is wasn't necessary.

Yes, exactly.

gav1nnn, I did some search on internet, too, when my house was to be painted. And yes, there is no mentioning about sanding. Which means it is not necessary.

Why you, after doing similar research and finding the same information

1) don't believe your painter?

2) ask the question here and when a professional decorator tells you it is not necessary, you don't believe him either?

 

[gav1nnn]

<sigh>

Because there were a number of things about how he was doing the job that seemed to be cutting corners (e.g. he painted the ground floor while his collegue scraped the cills upstairs raining dust and paint flakes onto the new paint), and when I asked about sanding (I am used to sanding interior surfaces before painting, but have no experience of painting external masonry) he couldn't explain to me why it wasn't necessary (unlike, for example, repainting the outside of window frames, where he said he would sand). Instead he blustered and changed the subject. He also said the job would take two days and then claimed to have everything but a second coat of paint done in 4 hours.

Given the cost to the family finances I thought I would confirm to my own satisfaction what the correct procedure should be, and as it happens his approach is correct.

The fact that published guides didn't mention sanding because it isn't necessary (I have not seen that stated explicitly anywhere) makes sense with hindsight, but I wasn't sure at the time so asked for advice.

 

[gav1nnn]

Oh, and my follow up to Dec wasn't mean to imply that I didn't believe him - my apologies if that was how my post read. I was hoping to understand why sanding is not necessary.

 

[TheDec]

Oh, and my follow up to Dec wasn't mean to imply that I didn't believe him - my apologies if that was how my post read. I was hoping to understand why sanding is not necessary.

Worry not, I took no offence. As a matter of fact I fully understand your situation, when you have to part with a large sum of money it is your right to know that the advice which was offered to you is correct.

Dec

 

[dcdec]

In my opinion.

Its not necessary to rub down sound external masonry because 1. Its not a fine finish such as you would see on interior walls. 2. exterior emulsions are generally flat (particularly when weathered) so no mechanical key is needed to promote adhesion.

The important thing is that the substrate is clean and free of fungal growth before application of paint.

..... and your decorator should no better than to prep above wet paint, but unfortunately there's a lot of bad painters around at the moment with the recession and all.

 

[dcdec]

just had a quick look through the dulux thing> As far as i can see it refers to rubbing down OR dusting off. Externals would only ever be dusted off, the literature does not specifically say rubbing down external render as far as i can see, it uses the phrase rubbing down/dusting off which would be interpreted correctly by a pro decorator as dusting off