Architects memo no. 34: February 1984
Rust revisited
It is fundamental that, for any one type of paint, the better the surface preparation the longer the applied system will offer protection. Where the removal of rust is neither practical nor economic the use of oily, penetrative, slow drying primers in order to encapsulate and seal off the rust has generally been recommended in the past. Topcoats are then used to try and build up an effective barrier.

Recent times have seen the popularisation, both overseas and in New Zealand, of mastic coatings for the treatment of rusty steel. The principles behind these coatings is to produce at once something that is of low molecular weight and highly penetrative, yet also provide a high build barrier effect. Major cost savings can be achieved in application costs by these one coat systems.

The most favoured vehicles for this type of coating are low molecular weight epoxies cured with mixed polyamide/amines. These materials can produce the necessary penetration, corrosion resistance, and build. The well known early chalking property of epoxies is overcome by the use of leafing aluminium pigments. Aluminium provides a complete UV screen for the epoxy as well as adding substantially to the barrier properties of the coating.

The Resene offering in this area is Resene Alumastic high build aluminium/epoxy mastic. This product has been formulated with a unique blend of resins, penetrants, and surface active agents that allows excellent wetting and penetration of rusted steel surfaces. Reaction occurs on the surface, setting the components into a high molecular weight, tough durable polymer. The ease of application belies the very high volume solids content of 90%.

A significant evaluation tool used during the development of the product was the salt-fog chamber. The very harsh environment created in the chamber probes any weakness that may exist in the coating system. Photographic evidence of the performance of Resene Alumastic against market standards is available.