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Aluminium box section for outdoor frames in the UK

Updated: October 2026

The bar is going outside as mill finish, for a garden screen or a light rack. Aluminium box section for outdoor frames can live in a UK garden because it forms oxide instead of red rust, if you keep rain out of the bore and keep bare steel and copper off the metal. Every bar on the [aluminium rectangular tube comparison](/hub/rods-and-pipes/aluminium-rectangular-tube) is mill finish, 800 mm or 1,000 mm, with no working load. Those listings were read on 16 October 2026 and no sample was left outside. A qualifying purchase through our links can earn a commission at no extra cost to you. Cap the ends, and do not treat a 1 m cut as a longer rail.

The bar is going outside as mill finish, for a garden screen or a light rack.

Our top picks

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm

Best overall

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm
6.6/10about £12.53
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Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 1 m

Rank 2

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 1 m
7.8/10about £13.15
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Aluminium Online Aluminium Online 30 × 20 × 2 mm

Best value

Aluminium Online Aluminium Online 30 × 20 × 2 mm
5.2/10about £11.00
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On this page
  1. 1.What mill finish does in UK weather
  2. 2.Rain in the bore
  3. 3.Fixings and mortar
  4. 4.Leave it, paint it, or send it away

What mill finish does in UK weather

Mill finish is the bare extrusion. Outdoors it dulls as the oxide grows, which is why it does not form red rust. It is not a marine grade. Finished-frame care sheets ask for closer checks within about five miles of the coast. These tubes are not those coated frames. Sea air, road salt, a wet steel bracket, copper, and fresh mortar can pit the film. Only the two Fabal bars state 6063-T6. If you are still choosing steel, that comparison is on the aluminium box section vs steel box section page.

Rain in the bore

An open end holds water. A horizontal rail becomes a trough, and a vertical post holds whatever runs to the bottom. The five listings include the tube only. Match a separate plastic cap to the outside size and the wall, and fit it after you have deburred the cut. A cap for 30 × 20 mm will not seal 30 × 15 mm. 1st Choice Metals and DC Iron sell push-fit caps sized to the outside. Hulamin describes white powdery stains where moisture is trapped against the metal, and says light stains need not change mechanical properties if the metal can dry. A small weep hole at the lowest point stops a sealed rail becoming a trough.
Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm

Testsieger

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm
6.6/10about £12.53
View offer

Fixings and mortar

Ordinary steel screws in a wet aluminium wall mark an otherwise sound frame. The British Stainless Steel Association notes that a small stainless fastener in a large aluminium part is often acceptable, and that a marine example pitted where insulation was missing. Use stainless fasteners, or isolate steel brackets with a plastic washer, and keep copper off the bar. Fresh mortar is alkaline, so keep the cut end out of the wet mix. On 1.3 mm aluminium a through-bolt or a rivet is more honest than a self-tapper. The size for the cap or the bolt is on the aluminium rectangular tube sizes page.

Leave it, paint it, or send it away

You can leave mill finish outside and accept that it will go dull, or paint it with a primer made for aluminium. A steel primer is the wrong can. None of the five bars arrives coated. Anodising is a plating-shop job, realistic only for the two bars that state 6063-T6. The longest bar here is 1,000 mm. A gate wider than that needs a joint or a longer buy. Cut and deburr before you cap, as on the how to cut aluminium box section page. If the frame will carry a person, a roof, or a floor, stop.

Common questions

Will mill-finish aluminium rust in the garden?
It will not form red rust. It dulls as oxide grows, and it can pit where salt, wet steel, copper, or fresh mortar attacks the film. A review that says the bar should not rust is talking about red rust, on a short file that is not a weather test. After the first wet winter, look at the fixings and the open ends, not only at the faces.
What is the white powder?
It is often a water stain where moisture sat on the metal, not the red rust of steel. Hulamin says light stains of that kind need not change mechanical properties, and that drying stops them growing. Thick crust at a steel bolt, or in a bore that stays wet, is a different problem. Dry the bar and fix the trap. Do not treat a mill acid dip as a garden recipe.
Should I cap the ends?
Yes, if the bore would hold rain. Buy caps separately and match them to the outside size and the wall. None of the five tube listings includes them. A weep hole at the lowest point stops a sealed rail becoming a trough. Fit the cap after you have filed the cut, or the burr will hold it off the end.
Can I use ordinary steel screws?
They will hold, and in the wet they can mark the aluminium. Prefer a stainless fastener, or isolate a steel bracket. BSSA’s note is that a small stainless fastener in a large aluminium part is often acceptable, with pitting where insulation was missing in a marine case. A 1.3 mm wall still prefers a bolt or a rivet to a screw that must cut its own thread.
Can I set the bar in wet mortar?
Keep aluminium out of fresh mortar. The mix is alkaline and it attacks the film where you will not see it once the bed has hardened. Sleeve the end, or wait until the mortar has cured. A mill-finish tube is not a fence post you concrete in and forget.
Is 1 m long enough for a gate or a greenhouse side?
Usually not as a single rail. The longest listing here is 1,000 mm, and a gate or a greenhouse side is often wider. Join two cuts only if you accept a weak joint, or buy a longer bar. Five stars are not an outdoor test. The 60 × 40 mm row is 5.0 from one review that says “as described”, and its technical block also says Sheet.

Related pages

Best aluminium rectangular tube

Aluminium rectangular tube

Aluminium rectangular tube sizes, wall and cut length

How to cut aluminium box section with a metal blade

Aluminium box section vs steel for a light UK frame

Sources

Public UK sources and amazon.co.uk listings from the category digest, checked on 16 October 2026. Shelf prices and star lines can change after that date.

  1. Consumer Rights Act 2015 (retrieved 16 October 2026)
  2. Consumer Rights Act 2015, section 9 (satisfactory quality) (retrieved 16 October 2026)
  3. HSE: welding (retrieved 16 October 2026)
  4. HSE: COSHH basics (retrieved 16 October 2026)
  5. Approved Document A: structure (retrieved 16 October 2026)
  6. Using the UKCA marking (retrieved 16 October 2026)
  7. BS EN 755-2: mechanical properties of extruded aluminium (retrieved 16 October 2026)