What is a Rust Inhibiting primer and how does it work?

Rust inhibiting primers help to slow the future formation of rust if a corrosion resistant coating system is damaged.  The term ‘rust inhibiting’ is normally used to describe primers that contain Zinc or Zinc Phosphate.

A Zinc Rich primer forms a sacrificial layer on the metal substrate.  Zinc is more reactive than iron and forms a sacrificial zinc oxide in the presence of moisture, stopping moisture and air reaching the metal below.  Zinc Rich primers should be specified when rust is very severe or where a higher level of protection is required.

Zinc Phosphate particles overlap to form a protective barrier which halts moisture reaching the metal substrate.  They are ideal for spot priming metal roof sheets prior to application of a new refurbishment coating.

It is important to understand that rust inhibiting primers do not kill existing rust – they inhibit rust forming by stopping moisture, thereby checking existing surface rust from progressing if the protective coating is compromised.

If it is impossible to remove all the rust satisfactorily, then once all the loose or flaking rust has been removed, the remaining hard ingrained rust can then be treated with a rust bonding/converting primer which will help to passivate the rust.  This can then be overcoated with a Zinc phosphate primer.

Why does rust make a coating bubble and delaminate?

Contrary to appearances, it is not the coating system that delaminates – it is the rust itself that delaminates, taking the coating system with it.  As rust is a progressive and destructive reaction, if a coating system without a rust inhibiting or sacrificial primer is damaged, allowing moisture to spread underneath, the oxidation process will commence and the rust will flake off.  This gives the appearance of ‘rust bubbles’ or flaking.

So can a rusty metal be coated effectively to halt the spread of corrosion?


Once all the existing loose rust has been ground away to a solid substrate, the use of a rust inhibiting primer or paint system will effectively slow the future formation of rust on the substrate.

If there is extremely severe rust present on steel, the application of a rust bonding primer will help to passivate the rust, prior to application of a zinc phosphate primer.

As with all paint applications, the better the surface preparation the longer the paint system will last.  The earlier corrosion is treated, the greater the chances of best results.

Contact the Rooflock Technical Team today who can assist with a wide range of metal coatings for roofs, gutters, walls, structural steel and many other applications.  Many of our systems are applied by approved contractors, thus ensuring a high quality installation.