[ Painting ]
[ Painting ]
Guidelines for painting stainless steel
When considering painting stainless steel , the initial question arises : why paint it in the first place ?
Text by ASSDA
If the design choice is simply about colour or aesthetic purposes and the product form has a smooth surface ( i . e ., sheet or round products ), using factorysupplied coloured stock attached to fasteners might be the preferable option . This construction method is commonly used for non-contact areas such as walls or facades , as exemplified in ASSDA FAQ 12 : Coloured and patterned stainless steel . Similarly , in new constructions , there are often reflectance limits to consider . While highly polished stainless steels offer extreme reflectivity , embossed surfaces , like the panels seen on the Petronas Towers in Kuala Lumpur , Malaysia , provide a bright appearance with lower reflectivity .
Clean profiled surface needed However , if adding colour is necessary for an existing structure , painting stainless steel is feasible but requires certain precautions . Unlike carbon or galvanised steel , a primer undercoat will not form a chemical bond with the stainless steel surface , necessitating a clean and profiled surface to provide a
Stainless steel general waste and recycling bins at the University of Queensland .
University of Florida ( UF ) Health Sands Children ’ s Hospital Photo : World Stainless .
mechanical key for adhesion . Profiling by chemical etching is possible , but safety concerns make using pickling acids impractical . A relatively mild etch profile can be achieved using oxalic acid , but the most common method of profiling involves abrasion either by blasting or using abrasive belts with aluminium oxide grit , which MUST be free of carbon steel . Garnet is used for blast profiling of carbon steel , but it is not recommended for stainless steel because of potential contamination from this natural mineral . Before abrasive profiling , the surface must be free of grease and dirt , and any heat tint from welding or thermal cutting must be removed by pickling or abrasion . Due to surface tension , liquid coatings tend to draw back from sharp edges . This requires rounded edges as specified for painted carbon steel ( refer to AS / NZS 2312.1 Protection of steel against atmospheric corrosion by the use of protective coatings : Section 3.4 ). In addition , if thin lacquers are used for an aggressive environment to reduce maintenance cleaning compared to a bare stainless surface , then a spider web of filiform corrosion can creep from sharp cut edges along the stainlesslacquer interface . For small projects , it is not uncommon to use abrasive belts , with typically 80 grit size . If blasting is possible , it should provide a profile of 30 to 50 µ m Ra for an inert primer , typically a two pack epoxy , with thickness at least three times the profile to ensure peak coverage and a similar thickness top coating of a UV-resistant formulation for exterior applications .
Choose coatings carefully The coatings must contain less than 50 ppm of available chloride . It is critical that the primer is inert , as zinc-based primer systems will galvanically react with the stainless steel and degrade the coating . Other inhibitors may add expense without enhancing corrosion resistance for the stainless steel . These stainless profiles are much greater than the 0.5 µ m Ra value for uncoated stainless steel because the surface profile for painting is about the adhesion of an aesthetic / protective coating rather than avoiding corrosion of a bare stainless steel . With the right process , the successful application of paint on stainless steel will deliver both aesthetic appeal and enduring protection against corrosion , ensuring longevity and visual appeal of the structure .
Republished with the kind permission of the Australian Stainless Steel Development Association ( ASSDA ), a non-profit industry group that has been developing a strong and vibrant stainless steel industry in Australia since 1992 . For information , visit www . assda . asn . au
30 Stainless Steel World September 2024 www . stainless-steel-world . net