Roof lantern frame finishes and powder coating

What polyester powder coating is, and how it gets onto aluminium

Powder coating is a dry finish. There is no solvent and no wet paint. Finely ground polyester resin, pigment and hardener are given an electrostatic charge and sprayed at an earthed aluminium extrusion, so the particles cling to the metal by charge alone, including round the back of the section where the gun is not pointing.

The coated extrusion then goes through an oven at roughly 180 to 200 degrees, where the powder melts, flows out into a continuous film and cross-links into a thermoset. Once cured it cannot be re-melted, which is why a powder coat cannot be reflowed to remove a mark the way a lacquer can. The finished film is typically 60 to 100 microns, which is around a tenth of a millimetre, and it has to survive twenty five Gloucestershire winters at an angle of 25 degrees with nothing shading it.

Pre-treatment decides whether the coating stays on

The coating itself rarely fails. The bond between the coating and the aluminium is what fails, and that bond is created before any powder is sprayed.

The extrusion is degreased, then etched in an alkaline or acid bath to remove the natural oxide layer and take a controlled amount of metal off the surface, then rinsed in de-ionised water, then given a conversion coating that leaves a microscopically keyed, chemically active surface for the powder to grip. Traditionally that conversion was chromate based. Current practice across the industry is chrome-free, using titanium or zirconium chemistry, which performs as well when it is controlled properly and is far less unpleasant to run.

None of this is visible on a finished frame. It is the difference between a coating that is still intact in twenty years and one that lifts in flakes from an edge after eight, and it is the main practical reason to ask which coating standard a frame is finished to rather than only which colour.

Architectural class one and class two polyester, and how each weathers

Powders are graded by how they hold gloss and colour under accelerated and natural weathering. The grades are what separate a frame that still looks like itself in fifteen years from one that has gone dusty and flat on the south face.

Standard architectural polyester, the common class one grade, is the default on lanterns and is genuinely good. Superdurable polyester, sometimes called class two, uses a different resin chemistry and holds gloss and colour substantially longer under ultraviolet exposure. On a vertical window frame the difference takes decades to become obvious. On a lantern it does not, because a lantern’s rafters face the sky at 25 degrees and take far more ultraviolet per year than anything on a wall. Superdurable is the sensible upgrade on any south or west facing lantern, and it costs a modest amount on a frame that is being coated anyway.

Film thickness in microns, and what happens below sixty

A coating specification names a minimum film thickness, commonly 60 microns for architectural work, and a competent coater will run at 70 to 80 to be safely above it everywhere.

Thin film is the commonest quality problem, and it appears in predictable places: the sharp arris on an extrusion, where electrostatic charge concentrates and powder is thrown off rather than attracted; the inside of deep channels, where the charge is shielded; and the back faces of complex sections. Where film is thin at an arris, that is where the first corrosion and the first flaking will start. On a lantern the arrises along every rafter cap and along the eaves beam are the exact edges that shed water and take grit, so a coater who runs to the middle of the range rather than the bottom of it is worth having.

RAL, BS 4800 and the reference that has to be on the order

Colour is specified by reference, not by name. RAL is the usual system on aluminium: RAL 7016 anthracite grey, RAL 9005 jet black, RAL 9010 pure white, RAL 7015 slate grey. BS 4800 references appear on some specifications, particularly where a job is matching existing joinery.

Two practical points. First, a colour name without a number is not a specification. Anthracite is not a colour, it is a family, and three suppliers will give you three greys. Second, the same RAL number in different gloss levels or from different powder manufacturers can read differently side by side, particularly in greys and off whites. Where a lantern has to match bifold doors already installed, get the RAL number and the gloss level from the door supplier and ideally have the lantern coated at the same coater, because that removes the variable entirely.

Gloss level, and why matt reads better on a frame you look up at

Gloss is measured as a percentage reflectance at 60 degrees, and it changes the character of a frame more than most people expect.

A high gloss frame at 80 per cent throws hard highlights along every rafter, and on a lantern those highlights are directly in your eyeline from below. It also shows every ripple in the extrusion and every handling mark. A satin finish around 30 per cent is the common architectural choice and the safest one. A matt at 10 to 20 per cent gives the flattest, quietest frame, hides surface imperfection best, and on an internal face merges most readily into a plastered ceiling. On a lantern, where the frame is overhead and the sky is behind it, matt or satin almost always beats gloss.

Smooth, fine texture and coarse texture

Texture is separate from gloss. A smooth powder gives a flat, even film that shows fingerprints, handling scuffs and any unevenness in the underlying extrusion. A fine texture powder has a slight orange-peel or sanded quality that scatters light at a small scale.

Fine texture is the better choice on a lantern for a straightforward reason: the internal faces get handled by the installer, by the plasterer working the reveal up to the eaves beam, and later by whoever cleans the glass. A fine texture surface hides all of that and continues to look even. It also disguises the small variations in film thickness around a complex section. Coarse textures exist and belong on curtain walling rather than on domestic joinery.

Anthracite 7016 is the default, and the reasons are not only aesthetic

RAL 7016 has become the standard exterior colour on aluminium glazing in this country, and on a lantern it works for reasons beyond fashion.

Seen from a garden, a dark grey frame reads as a shadow line between panes rather than as an object, so the roof reads as glass. It also sits comfortably against almost every Gloucestershire elevation: Severn Vale red brick, Cotswold limestone, Cheltenham stucco and modern render all take a dark grey without argument, where white can look stark on stone and brown dates quickly. And dark grey holds its appearance under dirt better than white, which matters on a roof that will be cleaned rarely.

Black, RAL 9005, is crisper and more emphatic, and it shows dust and pollen more obviously. White is right on a painted or rendered rear elevation and looks wrong against Cotswold stone.

Dual colour frames, and the extra pass through the oven

The internal colour is the decision people regret, and it is a separate decision from the external one.

A dark internal frame overhead reads as a heavy grid across the ceiling, particularly in a north facing room where no direct sun ever lands on it to lift it. The same lantern with an off white or a light grey internal face merges into a white ceiling and the glass becomes the thing you see. The perceived difference is greater than a real 15mm difference in section width.

Dual colour means the inner and outer extrusions are coated separately before assembly, or the section is masked and run through twice. It costs a modest amount more and adds a little to the lead time. On a lantern, where you spend every evening looking up at the internal face, it is nearly always the right answer. Common combinations are 7016 outside with 9010 white inside, or 7016 outside with 9016 traffic white inside on a job where the ceiling paint is a bright white.

Anodising, and where it beats a powder coat

Anodising is not paint. The aluminium is made the anode in an electrolytic bath so that its own oxide layer grows thicker, typically 15 to 25 microns for architectural work, then dyed if a colour is wanted and sealed.

The result is part of the metal rather than a film on it, so it cannot chip or flake, and it is extremely hard. It holds up superbly on exposed work and it gives metallic silver and bronze finishes that powder cannot convincingly imitate. The trade-offs are that colour choice is narrow, batch-to-batch colour matching is harder than with powder, and a deep scratch cannot be touched in because there is no film to make good. On a domestic lantern powder coating is usually the better fit. Anodising earns its place where a metallic finish is specifically wanted or where a frame is very exposed.

Marine grade specification within reach of the estuary

Gloucestershire is inland, and for most of the county a standard architectural coating is correct. The lower Severn is a different matter. Salt-laden air travels a long way up the estuary on a south westerly, and properties south towards Thornbury, Berkeley and the Severn crossing sit in a genuinely more aggressive environment than Cheltenham or Northleach.

The failure mode there is filiform corrosion, where moisture and chloride creep under the coating from an unprotected cut edge and lift the film in thread-like lines. The defence is a marine or seaside pre-treatment specification, sometimes with a primer coat under the topcoat, and attention to cut ends and drilled holes on site. Where a lantern is going onto a plot within sight of the estuary, that specification is worth naming on the order rather than assuming the standard product covers it.

Chalking, fading, and how a dark coating ages at 25 degrees

Powder coatings age by chalking: ultraviolet breaks down the resin at the very surface and leaves loose pigment behind, which reads as a dusty, flat, slightly lightened finish. Rub a finger along an old coating and it comes away with colour on it.

Two things accelerate it on a lantern. The first is angle: glazing bars at 25 degrees receive close to perpendicular summer sun for hours, where a vertical frame never does. The second is colour depth. Deep, saturated colours and blacks show fade far more than mid greys, because there is more pigment to lose. This is the concrete argument for superdurable powder on a south or west facing lantern in a dark colour, and it is a specification decision that costs little at order and cannot be revisited afterwards.

Cleaning a powder coat, and what takes the surface off

Maintenance is genuinely light, and the failures are all caused by the wrong product rather than by neglect.

Warm water and a mild detergent on a soft cloth, twice a year on an accessible frame, is the whole routine. What must not go near it: solvents, cellulose thinners, abrasive creams, scouring pads, and pressure washing at close range, all of which either dissolve the surface or cut through the film. Lime or cement splashes from adjacent stonework get washed off while wet rather than scraped off dry. On an internal face, the same rule applies, and kitchen degreasers are the usual culprit on a lantern over a hob, because they are formulated to attack exactly the kind of organic film a polyester surface presents.

Handling damage, touch-in, and a scratch that cannot be polished out

A powder coating is thermoset, so a damaged area cannot be reflowed. Site damage is made good with an air-drying touch-in paint matched to the RAL reference, which is a colour match rather than an invisible one, and it will always be slightly different in gloss.

The practical consequence is that a lantern is protected until it is glazed. Sections stay in their factory wrapping until the moment they are set, the eaves beam is bedded onto a clean kerb top, and glass is handled with pads rather than dragged across a coated rafter. The other habit that matters is cutting: any section cut or drilled on site leaves bare aluminium at the cut face, and that face gets sealed rather than left, because it is the starting point for corrosion under the film. On an assembled lantern the visible frame is one continuous surface and a single scratch across a rafter is in your eyeline for the life of the room, which is why the wrapping stays on as long as it possibly can.

Heritage colours on a Cheltenham or Cotswold rear elevation

In a conservation area or on a listed rear elevation, the finish is frequently a planning condition rather than a preference, and it is worth knowing what tends to be asked for before a frame is priced.

On the Cheltenham terraces through Lansdown, Montpellier and Pittville, and around the wolds villages at Painswick, Bibury and Chipping Campden, what commonly gets consented is a dark heritage colour on a section that reads as joinery: RAL 7021 black grey, RAL 6009 fir green or a specified BS reference matching existing windows, in a matt or satin finish, on a painted timber or timber-cored frame rather than a bright aluminium one. Bright silver anodised and white are the two that most often attract an objection on a historic elevation.

Because a non-stock colour adds to the lead time and the specification may be conditioned, the order of operations is pre-application advice first, colour agreed second, frame ordered third. Doing it the other way round is how a frame with a long lead time ends up being the wrong colour on the day the scaffold goes up. What moves the figure on a lantern job is set out under costs, Severn covers Gloucester and thirty miles by road around it on the areas pages, and a first answer starts from an email to info@severnskylightsgloucester.co.uk with a photograph of the elevation and any existing frames the lantern will have to sit alongside.