Saponification is a chemical reaction between an alkaline surface, moisture and the oils in solvent-based paint, producing a soft, tacky, soap-like film on top of the coating. It shows up most on soffits, fascias and metalwork that sit close to fresh masonry, cement render or new mortar joints.
It can, if the alkaline moisture source is still active — for example, new masonry that has not finished curing. Once that settles down the reaction usually stops.
Not always. Light cases wash off and recoat well. Long-standing cases where the film has softened right through may need taking back further.
It is far less common, since the reaction is specific to the oils in solvent-based coatings, which is one reason we often move to a water-based system in exposed spots near masonry.
Fresh mortar, cement render and new masonry are highly alkaline while curing. Rain washes alkaline moisture down onto nearby painted timber or metal, and where that meets an oil-based coating, a reaction takes place that leaves the paint feeling greasy or soapy rather than hard.
We wipe the affected area with a damp cloth — a soapy, slippery residue that lifts off is a fairly clear sign, alongside the location next to new masonry work being a strong indicator on its own.
The affected film is washed thoroughly with clean water to remove the reacted residue, allowed to dry fully, and then recoated. Where the reaction has continued for a long period the coating underneath may itself have softened and need taking back further before repainting.
Send us a photograph of the affected soffit or fascia along with any recent masonry work nearby.
A soapy film on oil-based paint near fresh masonry is a known chemical reaction, not a random paint fault, and it is fixable once identified.
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