Identify the surface and follow the current instructions for your exact products. Do not sand or strip uncertain old coatings. Read the safety guide before disturbing materials.
Think in layers with a purpose
A primer prepares a substrate for the next coat. Depending on the product, it may improve adhesion, manage absorption, protect metal or isolate a particular stain. An undercoat helps build an even base, opacity and sometimes the required colour beneath the topcoat.
Some products combine roles; others do not. A bonding primer for a glossy surface is not automatically the correct product for resinous knots, water staining or bare steel. Read the allowed substrates and exclusions, not just the front of the tin.
Match the problem to the system
Bare timber, MDF edges, galvanised metal and previously painted plaster have different needs. A manufacturer may specify a dedicated primer, a diluted first coat, a combined primer and undercoat, or no separate primer over a sound compatible coating.
For a stain, solve its cause first. A stain isolator can stop certain marks bleeding into paint; it cannot stop a leak. Check both stain coverage and compatibility with your intended finish.
A primer still needs preparation
Even a high adhesion product needs the cleaning and surface preparation in its instructions. Grease, dust and loose paint remain weak links.
- Identify the substrate, retained coating and the problem to solve.
- Choose a primer and finish confirmed to work together.
- Clean and prepare, then make a small trial where appropriate.
- Apply at the specified coverage. Do not thin unless instructed.
- Respect the drying and overcoating window before applying the next layer.
Common mistakes
- Treating primer as permission to paint over grease.
- Mixing products without checking compatibility.
- Using a water based primer for every stain without reading its limits.
Unknown old coatings, active water ingress and extensive corrosion need investigation rather than a stronger primer.
Sources & further reading
Original general guidance from Aethos, informed by the resources below. These references cover relevant safety, materials or techniques, not a specification for your property. Product examples are not paid recommendations.
- Zinsser: Bulls Eye 1 2 3 technical information (opens in a new tab)
An example of why preparation, substrate suitability and recoat instructions are product specific.
- Zinsser: AllCoat Exterior Satin (opens in a new tab)
A product example covering exterior substrates, preparation and application conditions, not a universal recommendation.
- HSE: lead in construction (opens in a new tab)
Why disturbing older lead paint requires a planned, controlled approach.