Paint flaking/bubbling/wrinkling on exterior sill within months. Where am I going wrong please? Sandtex smooth paint used.

[bigronmanager]

 

[freddiemercurystwin]

What prep did you do beforehand? You know the less information you add to your query the less quality answers you'll receive.

 

[bigronmanager]

Thank you. I just scraped off the old flakey layers of paint that were loose and put a couple of layers of the Sandtex on. It had been a very dry few weeks before applying.

 

[JohnD]

Were you painting onto bare stone? Onto old oil paint? Did you wash off the dust?

You can scrape off some of the wrinkles and see what's behind them, and if the paint has lifted an old coating. It looks to me like the paint had not adhered to the substrate and stretched, perhaps when it got wet. That sort of thing happens if you paint over glue (usually seen indoors).

Did the sun shine on the paint while you were working, or while it was drying?

 

[bigronmanager]

Thank you. It was pretty sunny. The old paint was pretty glossy that I painted over. I brushed off the dust from the scraping.

 

[JohnD]

Scrape a bit off and see what is under the wrinkles.

I don't think masonry paint will stick to gloss.

 

[DiyNutJob]

Moisture and biology can cause failure. Napalm the substrate with undiluted bleach using brush, dry, optionally clean with water, dry, then prime and paint. Nothing you can do about moisture present. Wait for the paint to fail and spot repair. Look for places where water can get below paint and seal. All paint edges should be examined. Cracks should be filled. Hybrid polymer sealant will let you fill the smallest of cracks.

 

[sparkwright]

As mentioned, either you've painted on a damp surface, or you haven't sanded down an old gloss finish.