Why two walls use different amounts of paint
New plaster, brick, mineral render and an already painted surface don't have the same porosity, cohesion or absorption. When a base absorbs unevenly, drying and the spread of the finish can vary from one area to another. Surface preparation aims to reduce these differences for a more controlled application.
What a primer does
A suitable primer can regulate absorption, consolidate certain bases and help the next coating bond, depending on its formulation and the intended surface. It isn't a universal adhesive and it won't fix a wall with active damp or crumbling render. Before choosing one, check for dust, old coatings, salts, whether the surface is mineral or synthetic, and curing time.
Sellador Total L44
Sealing primer: it evens out the absorption of the substrate and seals the wall before painting.750 ml, 4 L and 15 L · 9–10 m²/L per coat
Adhesion: a property of the whole system
The strength of the finish doesn't depend on the topcoat alone. Failure can occur at the surface-primer bond, between primer and finish, or within an old coat. ASTM D3359 is a reference for testing the adhesion of some coatings, but its results don't replace identifying the cause of detachment on site.
Sellador Total L44 in Humix Paint's documentation
Sellador Total L44 is a clear, water-based 100% polysiloxane sealer for indoor and outdoor use before painting new, absorbent or weak surfaces. The data sheet states it regulates absorption, improves adhesion of the finish, optimises its consumption and preserves the breathability of the substrate. It isn't an anti-mould or biocidal treatment: if there's already mould, it has to be removed first and the cause of the damp corrected. It should be used diluted 1:1 with water as a primer or neat as a sealer.
Application details that matter
The data sheet allows application by spray, brush or roller and allows a second coat or finish after 24 hours. It also states that mineral substrates must be at least 14 days old and that the application temperature must be between +5 °C and +40 °C. If the surface has salts, water ingress or lifting paint, these must be dealt with before the surface is considered ready.
Conclusion
Priming isn't about adding a coat for the sake of it. It's about matching the system to the real state of the surface so the finish goes onto a suitable base.
Technical and scientific sources
- ASTM D3359-23, Standard Test Methods for Rating Adhesion by Tape Test
- Whole Building Design Guide, Moisture Management
- EPA, A Brief Guide to Mold, Moisture and Your Home
Guía escrita y revisada por el Equipo Humix Paint con los datos de las fichas técnicas de cada producto. La pintura actúa en la superficie: si el agua sube del suelo o entra de fuera, primero se trata la causa.
