What Makes a Good Metal Paint?

Metal Paint covers a whole spectrum of coating technologies from the simplest 3 coat system favoured by traditionalists of a primer, undercoat and finish coat to the products favoured by the DIY market as single pack, direct to metal coatings (also known by their finish as “smooth” or “hammer finish”) to high end epoxy finishes and polyureas. Recently water based products have also entered the market but so far the performance has not lived up to the dream and the basic chemistry – iron needs two basics to corrode – water and oxygen – both of which you find in abundance in water based coatings and even the best so far do not match the performance of a well formulated solvent based or solvent free coating (yet!). In fact even the type of metal has an impact on your choice of coatings. Iron, steel and variants thereof corrode rapidly in the presence of water and oxygen by forming the well know hydrated iron oxide – rust! So for these metals specific chemistry is used to combat this in the form of barrier pigments, highly cross linked polymers and anti- corrosive pigments to inhibit the formation of the oxides which cause blistering and coating failure. Other metals found in general use fall broadly into the category of non-ferrous metals (Zinc hot dipped, galvanised, plated or in the form of aluminium or brass / copper) and these different metals all have a different electrochemical corrosion potential. In  simple terms – corrosion is a battery with the metal losing material at the anode (pitting) and metal ions “joining” the solution and being deposited at the cathode as salts

However, for a really efficient metal paint the choice of anti-corrosive pigment is also very important, some would argue the most critical aspect of the paint. Historically red lead was used and whilst this did indeed work it was found to be dangerous to health and eventually the government introduced lead at work regulations which have almost eliminated this technology. Higher performance products were introduced more than 50 years ago, originally based on zinc oxide derivatives such as zinc phosphate and zinc chromate and indeed even today on many older specifications people still ask for BS5493 zinc phosphate paint – even though this has been superseded by much more efficient and safer technologies. Higher specification products were based on derivatives of zinc and chromium but these derivates like zinc chromate, zinc tetraoxychromate and zinc potassium chromate all bring serious health risks all are carcinogenic and all are to a greater or lesser degree water soluble so easily migrate to the water supply. This in combination with progress on corrosion testing and the replacement of the highly ineffective B117 salt spray test with more realistic testing techniques including prohesion and cyclic testing with QUVB lead to the development around the world of several non-toxic environmentally friendly anti corrosive pigments for the metal paint formulator -the early products were based on calcium ion exchanged silica gel but these proved to be temperamental and were slowly replaced by alternative phosphates and modified phosphates based on nontoxic metals including calcium and magnesium sometimes in conjunction with other inhibitors  - one of the market leaders today in this technology is Nanoguard ACP and there are similar older generation versions of this technology still used in metal paints around the world. Nanoguard ACP was however, unique in its ability to offer a totally non toxic, safe, heavy metal free solution for the formulator of metal paints. Other similar technologies became available from other suppliers as people tried to copy the technology – with varying degrees of success. Metal Paint comes in a range of options as previously mentioned but generally today most people prefer to buy a product which requires fewer coats of metal paint, one with no “hazard labels” (toxicity, environmentally harmful etc) or at least based on an easily recognised renewable resource such as soya oil or sunflower oil. Yellowing resistance is also paramount in lighter colours of metal paint and for this reason most modern metal paints are also based on a synthetic modification but for those really looking for an environmentally friendly metal paint there are few that cancompete with a good short oil metal paint, based on sunflower oil and glycerine and based on a nontoxic environmentally friendly anticorrosive pigment such as Nanoguard ACP DCP.   As with most things price is not always a guarantee of quality and it is always worth checking those points if they are important to you when choosing your metal paint, particularly if children or animals are likely to be in close contact with the surface painted with your metal paint. Finally, metal paints should always be applied in accordance with the technical data sheet from the supplier to get the best results. It is also good practice unless buying a specially formulated metal paint which is surface tolerant to prepare the metal to be painted well before application of the metal paint. This usually entails removing al loose and flaking material (rust, old paint and any other contamination) immediately before application – ideally to standardST2 as a minimum for manually prepared surfaces. The surface should be clean and dry before application to avoid trapping under the paint any residues and this will ensure an optimum life for your metal paint and keep the surface looking good for the maximum possible time. Time spent on preparation and choosing the right metal paint always pays for itself on longer life and easier maintenance!