17 years ago in 1992 Altex Coatings Ltd was asked to provide a maintenance painting specification for canopy trusses on a cargo shed at the Port of Tauranga.

The cargo shed was constructed in 1967 and the steelwork, including the canopy structure, was protected with an unidentified bituminous paint system that after 25 years of service (including minor interim maintenance) had degraded to the point that red rusting of the canopy structure was widespread, estimated to be more than 25% of the exposed surface, however it was still deemed to be maintainable rather than having to be fully replaced.

Situated on the main wharf at Tauranga the canopy structure is within 100 metres of the inland port marine environment facing the prevailing westerly winds and only 500 metres from the easterly Ocean Beach at Mount Maunganui. This exposure is officially designated “Severe Coastal” and even allowing for rain-washing of the exposed canopy trusses the corrosion rate (steel loss rate) is estimated to be > 35 micrometres per year.

In other words left uncoated from time of construction to the present day the loss of steel would have been a minimum of 1.4 millimetres on exposed faces of hollow section members and over 2.8 millimetres on flats and angles. Degradation through ‘pack-rusting’ at flanged joints would have been catastrophic under these conditions.

Logistics and access dictated that the Port of Tauranga would need a ‘simple’ specification with easily handled products not compromised by weather or cargo handling delays for the basic maintenance system. In addition any nominated system would require a proven track record in similar environments and must also be self-maintainable in the long term.

Altex had extensive case history for a variety of coating systems for the Port of Tauranga; amongst these were several utilising a proprietary alkyd anti-corrosive primer, High Build Rust Barrier (HBRB).

Being a single pot product makes HBRB useful in both contract and ad hoc maintenance painting and thus it got extensive use throughout the port facilities. Standing out amongst these uses was the extraordinary performance of HBRB Primer and an alkyd topcoat on floating equipment such as pile driving barges.

It was therefore an easy step to gain confidence in HBRB to perform just as well on the cargo shed canopy structure.

The steel surface area of the canopy lattice trusses is approximately 800m2 and maintenance painting continued over several weeks in which expected logistics downtime took place as well as the occasional periods of high wind, sea-spray, and rain.

At the start of the maintenance project the canopy trusses were washed down however thereafter the only practical preparation was progressive spot abrasive blasting or power tool cleaning of corrosion and unsound existing paint.

The basic maintenance specification was kept as simple and practicable as possible, viz;

Wash and clean, spot prime with one coat of Altex High Build Rust Barrier and fully coat with a second and final coat of Altex High Build Rust Barrier.

Thanks to flexibility of use of the single pot HBRB and the excellent surface tolerance of the primer the contractor completed the work without any problems except for the anticipated weather or cargo handling delays.
Two years after completion of the canopy maintenance a regime was set up by Altex and the Port of Tauranga that the canopy would get an annual inspection and assessment.

Thus for the intervening time 14 annual inspections have been carried out in which the first 13 rated the condition of the paint-work as “near-perfect” at Rust Grade 10 and in the most recent survey carried out in 2008 the condition was rated as Rust Grade 9 – ie less than 0.03% surface rusting most of which is confined to sharp edges and ‘hidden’ areas which may have been compromised in the original works.

To achieve a level of protection that shows as little breakdown as Rust Grade 9 after 16 years in a severe coastal exposure is pretty good for even the most sophisticated coating systems; that a proprietary formulated alkyd primer designed and made by a local Kiwi firm could provide such protection in just two coats bears testimony to “keep it simple, do it right”.

As a post script it is worthy to note that Altex High Build Rust Barrier is the only primer of its specific type that has been approved for use on the Transit New Zealand owned Auckland Harbour Bridge.

Download the High Build Rust Barrier Datasheet or contact us for more information