Advice on making good external paint job

mct1

We had our house exterior painted last November. Mostly it's fine, but the paint on 8 windowsills of the new extension (despite being completed six months earlier) is flaking off in a major way.

It's not a big job so I'll do it myself. Will it be sufficient to roughly sand down the sills to remove loose paint and apply a couple of coats of Weathershield? Or do I need to apply some kind of sealant to prevent more flaking?

Many thanks.

Wertz

Firstly what sort of paint was on the eight windowsills prior to the decorating job last November?
If it was oil paint (gloss/eggshell or even undercoat) and the decorators just put masonry emulsion over it, then that's the reason for the flaking.
If there was masonry emulsion on them, how long had they been painted (from new)?
Pre-cast concrete when bare, needs a stabilising solution applied, or a proprietary primer to allow masonry paint to adhere properly.
I'm guessing it's an underlying coat of oil paint that is causing the problem though...

Your problem now is not knowing why it's flaking....you could get the surface back to a sound one with a scraper and sandpaper, and then apply a primer/sealer such as the products Zinsser make, then apply your weathershield over that, but if it's oil that's underneath, it's not guaranteed that the layers will stick for the time that they usually would; sunlight, moisture and frost take their toll on paint and if it's not adhering properly then water eventually gets underneath and the top layers blow or flake off...

mct1

It was bare new concrete so you'd think someone who does painting and decorating for a living would know that it needed a primer, wouldn't you? Unbelievable. Lots of prep for me then before I repaint.:rolleyes: Many thanks for the advice.

Wertz

TBH it shouldn't have fallen off or started flaking in so short a time even without the primer, unless they didn't even use a proper exterior paint....sounds like perhaps the sills were coated with interior matt maybe or something, or possibly undercoated with oil and then finished in masonry emulsion.
The stabilising primer whilst important for making the paint stick over a long period, shouldn't lead to paint failure in 12 months unless the sills are constantly wet.
As far as P&Ds knowing what they're doing? lol It's not even about them knowing what to do (many of them don't), it's the added expense and time of doing things the right way that leads to shortcuts like this and it doesn't become apparent until the job is long paid for...

To make thing easier/cheaper for you, at this stage, you can bring the sills back to sound surface with scraper and sandpaper, wash and dust them off then get a paint additive called E-B by Owatrol, that you mix with your first coat of weathershield and apply, then applying the 2nd coat as normal. Easier to find E-B than zinsser primers...