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Primed or Factory Finished Cast Iron Guttering?

We all recognise that short term gain frequently has expensive long term consequences. With cast iron rainwater systems, buying primed and painting it on-site versus installing fully-painted factory-finished product, the case for factory finished is, well… ‘cast iron’. The arguments and costs presented in this make factory finished a clear winner.

Cast Iron_bespoke2

Primed and then painted on-site using skilled labour costs 26% more than factory finished over its expected lifetime.

Primed and then painted on-site with unskilled labour costs 33% more than factory finished over its expected lifetime.

Leaving primed cast iron rainwater on-site exposed to the weather for the first five years, and painting it when it looks like it needs painting, is 69% more costly.

When it comes to buying and installing cast iron rainwater, there seem to be two schools of thought. Buy it primed (i.e. with only the manufacturer’s temporary ‘transit’ coating) then pay a contractor to paint it on-site. Or, buy it factory finished, pre-painted. The manufacturer’s instructions spell out the choice, as the following copy from Alumasc’s website makes clear:


1Primed Finish: Alumasc supplies cast iron products factory primed with one coat of protective grey oxide primer. This primer will give protection against corrosion during transportation and short-term undercover storage, and will provide a suitable surface for final painting. On-site handling and painting are the responsibility of the contractor.

2Pre-painted Factory Finished: The renowned quality of Apex Heritage cast iron from Alumasc is now available in a high performance satin pre-finish black. The finish is a three coat system which is factory applied under controlled conditions and has been specifically developed for cast iron to provide a smooth and long lasting finish.

But manufacturers’ statements and recommendations don’t make clear to specifiers and property owners what the implications are, particularly the knock-on effects and real cost of their choices. They don’t spell them out because it’s assumed that everyone knows and has thought about them. But generally people aren’t thinking about rainwater products and haven’t thought much about the consequences and costs of their choices.

You only really notice it when there’s a problem

For the most part, people don’t think about rainwater systems because of where it is on a building. Rainwater gutters, and the fascia and soffits they are fixed to, are used at the junction between roofs and walls. It’s a particularly exposed and vulnerable part of the building where extreme weather tests the weaknesses in a building. If water - the great destroyer of buildings - gets in, the consequences for the building can be severe. Yet we don’t often look at this part of a building. Our eyes tend to focus first on roofs or the windows and doors. We may not even ‘see’ where roof and walls intersect. And what we don’t see, we don’t think about. It isn’t the only reason maintenance gets skipped or delayed, but being out of sight and out of mind is a contributory factor. We tend to notice it when something is broken or looks like a problem. For example, we immediately see sagging or leaking gutters, running water or green slime on the walls, or seriously flaking paint or rust showing through.

3Protecting buildings from weather and water: Water is extremely damaging to buildings, and gutters, and downpipes are one of the building’s main lines of defence against water penetration. So when you see problems with rainwater products that are visible from the ground, things are usually serious. By then a lot of damage may have already been done.

If water does penetrate, it damages the fabric of a building, weakening it from damp and various forms of rot and mould that make it unhealthy to live in and potentially destroying its value. British buildings have to cope with strong winds and storms, extremes of temperature, and seasonal snow which can be heavy and potentially very damaging. Large volumes of rainwater fall on our roofs, sometimes in great bursts which have to be removed quickly in a controlled manner so we stay dry and secure inside. That’s what rainwater systems do, that’s what they are for. If they succeed, they contribute to sustaining the use and value of the building. If they fail, they contribute to the fall in use and value.

No one would choose to do any of this, of course. It’s the result of arm’s length decisions where the consequences may be separated in time from the choice, and where not having the right information makes it easier to choose apparent over real savings. If specifiers and property owners think about the choice between primed and fully factory finished rainwater systems, they may perceive the choices as price or a matter of convenience, roughly similar in impact and cost neutral to property maintenance. Nothing could be further from the truth. However, it’s hard to truly understand the adverse knock-on effects, or establish the real costs for poorly painted cast iron rainwater systems. This paper attempts to put that right.

Why cast iron?

Cast iron is a popular and well-used material for rainwater guttering and downpipes on many heritage and period properties in the UK. It is tough, incredibly strong, lasts for years and is 100% recyclable. If properly manufactured, protected, installed and maintained, cast iron will look good and enhance the properties it is protecting for over 100 years.

5It’s remarkably good at doing its job of protecting buildings: it’s attractive, economically effective and environmentally sustainable.  But if neglected and exposed to the elements without proper protection, cast iron will rust, flake away and crumble to nothing, disappearing in a process of natural recycling.

What protects cast iron?

The answer is paint. The paint system, type and quality of paint, number of coats, and how and when that paint is applied influences its durability, lifetime, and ongoing maintenance costs. Not all architects, specifiers, property owners, facilities managers, contractors and installers are aware of the implications of installing inadequately painted cast iron guttering, or of the maintenance considerations.

So they continue with current practice and opt for the cheapest, quickest or most convenient route. This includes buying cast iron rainwater primed only (i.e. with only the manufacturer’s temporary ‘transit’ coating), and then painting it on-site. They may even buy it primed and install it without painting it at all, assuming they get a few years’ protection from the primer coating. But that is to misunderstand what a primer is and what it’s designed to do. And it’s a seriously false economy. Not only will subsequent maintenance costs be significantly higher during the lifetime of the cast iron rainwater, but a poor start shortens its life and ensures it loses its looks and ages badly. And by failing in its primary duty to protect against water ingress, that may also have a serious impact on the building itself.

6Primed cast iron rusts rapidly on-site

Primed cast iron rainwater lasts for only a short period of time and should be painted immediately. The primer is factory applied and can be used as temporary protection for the product in transit. That’s it. Left unpainted and exposed to the elements, it will start to corrode and rust rapidly. If you are living in or responsible for a beautiful heritage building or period property, the last thing you want to see is rust stain running down the walls from your cast iron guttering. It will be expensive and difficult to fix. Rust is extremely difficult to remove because it eats into and creates corrosion pits in the iron substrate. Even after treatment with a brush and powerful anticorrosion chemicals, small amounts may remain in those pits, which become centres for rust to build and spread aggressively. Depending on the extent of rust and how it’s treated, it can necessitate a complete replacement much sooner than planned – a big expense.

Buy primed and paint on-site for equivalent results?

Not really. The option of buying primed cast iron rainwater for your contractor or installer to paint on-site may seem sensible, especially if you have paid for them to be on-site anyway. But the extent of protection will vary depending on the quality of their work: the thoroughness of preparation, the consistency and number of coats applied and most important of all the type of paint used!

Guttering that is primed and then painted on-site also needs to be protected from the elements before and during painting. Rust forms as tiny corrosion centres which are practically invisible to the eye. The longer it is left exposed, the greater the likelihood that rust will form and cause long term damage as a result. Even fast drying water based paint needs time to fully dry (cure or set) and achieve its complete protective properties. And in damp, humid or extreme weather, paint takes far longer to dry than in an environmentally controlled factory setting. While it is drying, paint is vulnerable to the weather, so painting should be done in the dry. But coming to the end of a project, when time has to be made up, the pressure to finish on time often overrides good practice. Labour is by far the largest cost in any installation job. So, the small material cost saving from buying primed only and painting the guttering on-site using expensive contractors’ labour far outweighs the perceived saving. It doesn’t stand up to logic. In fact the logic leans the other way: using labour-saving factory finished cast iron rainwater saves costly on-site labour time.

The best option: pre-painted factory finished

The best way to ensure cast iron rainwater remains rust free and beautiful for years, with minimal maintenance, is to install fully factory painted cast iron guttering. Alumasc’s fully factory painted cast iron for example is approximately 13% more expensive than primed cast iron, but it saves the labour cost of on-site painting; a much bigger cost. More significantly, the long-term cost savings far outweigh this modest initial investment.

56The Apex certified paint used on Alumasc’sHeritage cast iron rainwater is an advancedenvironmentally-friendly water based threecoat system, factory-applied under strictlycontrolled conditions. Not only does it saveon-site labour cost of painting, but the protective paint is applied to a far higherstandard so it performs better. Fully factorypainted cast iron rainwater can last up to 10years before it needs repainting. Dependingon exposure to the weather, it may need alight touch-up in 5-6 years to maintain the protection.

Alumasc’s Apex certified factory painted finish has been specifically developed to give a smooth, long lasting finish to cast iron. This is extremely difficult to replicate on-site. If surfaces are not are fully and evenly painted rust will set in, exposing weaknesses and putting at risk the entire guttering system and the fabric of the building it protects. The classic three coats in an external paint system are primer, under or second coat, and a top coat. The three coatings work together as a system with each binding together and doing different jobs in protecting the substrate.


Primer: The primary job of the primer is to adhere i.e. stick the paint system to the substrate and provide a key for the undercoat. The primer is usually a thin vulnerable coating that gives only slight temporary protection from the weather if used as a ‘transit’ coat to protect product on its way to site. It is good practice to keep the unprotected product inside or covered up, and to paint it as soon as possible.

Under or second coat: The primary job of the under or second coat is to adhere firmly to the primer or basecoat and provide opacity and a smooth firm base for the topcoat. The undercoat will be flexible enough to move without cracking as the metal rainwater expands and contracts in the heat and cold.

Topcoat: The topcoat adheres to the undercoat, protecting the substrate against corrosive effects of the atmosphere and weather. The topcoat will also look good, providing an attractive colour and finish that resists the build-up of dirt which gets washed away in the rain. The topcoat is sufficiently flexible to expand and contract without cracking as the underlying product expands and contracts in the heat and cold.

The topcoat also protects against the damaging effects of UV light, just as sun cream protects our skin. UV light causes brittleness, cracking and premature ageing in coatings. Normally paints use Titanium Dioxide (TiO2), the same protective agent we use in sun creams and sun blocks. The topcoat also withstands everyday minor knocks and abrasions which could damage the coating and expose the cast iron below to the atmosphere and weather.

Factory applied finishes are usually different in composition from standard site applied paints. It’s possible to use different and tougher two part systems for example, because the mix and application conditions can be controlled in a factory to give better adherence and a more uniform longer lastingfinish. On-site application is at the mercy of several variables such as the weather, the skill and care of the painter and the rough and tumble of a busy site. Alumasc uses a three coat factory applied system which includes a two-part epoxy primer underneath the top coat. An Apex certified label is added to each length of gutter or downpipe to confirm that the product has been coated under factory conditions.

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