Painting Over Old Gloss: Why Preparation Is Everything

The Problem with Old Gloss Paint

Gloss paint — or more precisely, the alkyd-resin oil-based gloss paints used on London woodwork from the 1960s through to the early 2000s — presents a specific challenge when it comes to repainting. Gloss is designed to resist water and abrasion. Those same properties make it resistant to adhesion from a new paint coat. Apply a fresh coat of paint over a sound, glossy surface without any preparation, and the bond between old and new is weak. The paint may look fine immediately. Within months, it begins to chip and peel at impact points — door frames, window sills, skirting boards — and within a year, the whole surface can be lifting.

This is not a theoretical problem. We see it constantly when called in to correct work done by contractors — or eager DIY homeowners — who skipped the preparation stage.

Keying Versus Stripping: How to Decide

There are two approaches to painting over old gloss: keying (abrading the surface to create a mechanical bond for the new paint) and stripping (removing the old paint entirely back to bare wood or metal). The correct choice depends on the condition of the existing paint.

Key the surface when:

• The existing gloss is sound, well-adhered, and not excessively thick (as a rough guide, if the moulding detail on architraves and skirting boards is still clearly readable, the paint build-up is not yet excessive)
• There are no areas of bubbling, flaking, or delamination
• The surface is not chalky or powdery

The keying process: sand the entire surface using 120-grit abrasive paper, working with the grain on timber surfaces. Flat areas can be done with a random orbit sander; moulded sections must be done by hand using a profiled sanding block or folded abrasive. The aim is to produce a uniformly dull surface — no shiny patches — which provides the mechanical key for the new coat. Wipe down with a damp cloth and allow to dry before applying primer.

Strip back to bare when:

• The paint is bubbling, flaking, or lifting in more than small isolated areas
• The paint build-up has reached the point where moulding detail is being lost
• There are signs of moisture ingress or wood rot beneath the paint film
• You want a factory-smooth finish for a high-specification room

Stripping methods: chemical stripping using a product such as Barrettine Paint Panther or Nitromors (gel formulation for vertical surfaces) applied and left to work, then removed with a scraper; or heat stripping using a hot air gun on flat sections, being careful never to use a hot air gun near glass or on lead paint. Always work with good ventilation when stripping chemically.

After stripping, sand to 180 grit, treat any exposed knots with shellac knotting solution (Zinsser Bulls Eye Shellac), and prime before any further coats.

Primer Selection: Getting It Right

The choice of primer matters more than most people realise when painting over problematic surfaces.

For keyed, sound gloss: Zinsser Bulls Eye 1-2-3 is our standard choice. It is a water-based shellac-modified primer that bonds to almost any surface without the need for mechanical key alone. Apply one coat, allow to dry (30 to 45 minutes at 20°C), sand lightly with 240-grit paper, and proceed to topcoat. Alternatively, an oil-based alkyd primer provides an excellent base and is compatible with all topcoat systems.

For bare timber after stripping: Treat knots with Zinsser Bulls Eye Shellac knotting solution first — a single coat will suffice on well-dried timber; two coats on pitchy or resinous timber. Then prime with an oil-based primer or Zinsser Bulls Eye 1-2-3.

For metal surfaces (window frames, balustrades, ironwork): A zinc-phosphate primer is the correct specification — it provides corrosion inhibition as well as adhesion. Zinsser Perma-White or Hammerite Direct to Metal primer are both reliable products.

Do not use a standard PVA primer or a diluted emulsion mist coat over gloss or bare wood. These are substrate sealers for plaster, not primers for timber or metal.

What Goes Wrong When Preparation Is Skipped

The failure modes are predictable:

Peeling at impact points is the most common result of painting over unkeyed gloss. The paint bonds adequately to the glossy surface in undisturbed areas but fails at any point that receives a knock or repeated friction — door frames, window sills, the bottom of doors. Within six to twelve months, the new paint is lifting in patches.

Fish-eye and crawling occurs when paint is applied over a surface that was not properly degreased. Kitchen and bathroom woodwork in particular accumulates grease and condensate residues that cause water-based topcoats to bead and crawl rather than flow out evenly. The result is a surface with small craters or ridges that cannot be corrected after drying.

Tannin bleed on oak or softwood that has been stripped back to bare — dark brown or orange staining visible through the topcoat. A shellac-based primer (Zinsser BIN) is the only reliably effective barrier against tannin bleed. Standard primers do not block it.

Paint build-up failure on surfaces where many generations of gloss have been applied without stripping: when the total paint film exceeds around 3mm, the stresses from seasonal timber movement cause the whole film to crack along all movement lines simultaneously, typically after the first cold winter following repainting.

Getting the Job Done Properly

Preparation for painting over old gloss is not glamorous and it is not quick, but it is the difference between a finish that lasts ten years and one that needs redoing in two. If you are commissioning a decorator to repaint your woodwork, ask specifically what their preparation process is before accepting a quote.

For a free written quote specifying the preparation and paint system for your London property, use our free quote page or reach us via the contact form.