Windows: paint inside, weather outside

Inside and outside are not the same tube

Inside you want a meeting with plaster or paint that can be painted and that shuts out air. A decent elastic acrylic or a paintable hybrid. Outside you want water, UV and serious thermal movement in the profile: a façade-grade neutral silicone or an exterior hybrid.

Interior acrylic in the sun dries out and goes. Painting non-paintable silicone gives beads of paint that peel. Mixing up glazing sealant (glass to bead) with the frame-to-façade joint is another classic: the first is not built for a wide gap between masonry and PVC or timber.

Tighter to air on the inside

Warm indoor air pushes vapour into the opening. If the inner joint is split, that vapour condenses on the frame and grows mould. So the inside is worked more continuous and tighter to air than the outside, which has to take expansion and rain.

In summer the profile can get very hot; in winter it shrinks. The swing on aluminium or PVC is huge. A skinny joint with no backer rod is overloaded.

When to seal — and when not to

Yes inside, paintable: frame-to-wall that will be painted.

No living-room acrylic outside.

Yes outside: façade-grade neutral or hybrid, elastic.

No if the sill is rotten, the subframe has dropped or there is an active leak in the cavity. Capping the outer joint will not dry that; the water finds another path.

How I do it

1. Strip leftovers, dust, algae and grease. Degrease, let it dry. Outside, not on wet brick.
2. In deep joints, PE backer rod. Two faces, not three.
3. Primer on very absorbent mineral if needed.
4. Nozzle cut to the joint width, steady pressure.
5. Tool it even.

On large openings the frame-to-masonry meeting usually wants about 10 mm of width, not a hairline. Façade movement classes (20LM / 25LM and the like) are typical product classes, not a promise for your job.

Exceptions

An exterior hybrid can be used inside, but if you will paint, choose paintable. Bathroom sanitary outdoors: mould resistance yes, UV and frost no.