Mar in haste; Repaint at leisure

Finishing Articles - Faults on Surface Finishes

Malcolm Griffiths looks at faults on surface finishes and how to prevent them. Since joining the paint industry in 1963, Malcolm has worked for several leading coating manufacturers. He is a graduate in chemistry, Fellow of the Institute of Metal Finishing and a Chartered Member of the Institute of Occupational Safety and Health. He now runs the independent coatings advisory service Ad Qual Castech Limited.

The old chestnut ‘How come we always have time to put it right but never have time to do it right first time?’ has never been more appropriate than in the coatings industry. The surface finish is the first thing anyone notices about a product and a fault in that finish can undo a sure-fire sale faster than anything else.

We are talking here about visual film defects that should be relatively simple to resolve, not the sort of faults that arise in service. The sort of problems we are referring to include disagreements over:

  • Physical damage arising from processing
  • Surface defects such as fish-eyes, pinholes, ciss-marks, bits and fibres
  • Colour variations
  • Gloss & flow variations
  • Handling, assembly or transit damage

The problems usually start through lack of foresight; a failure to agree what will be acceptable and what won’t. The importance of any particular defect will vary with the end use of the item: for example, a pinhole in the coating of a domestic chair leg may be unsightly but will be unlikely to cause long term problems in service. A similar defect on a part of the steering mechanism of a lorry could lead to early corrosion and catastrophic failure. The size and location of the pinhole can also be important factors. On a decorative finish, it may completely mar the appearance.

The trouble is that there are no absolutes in this. Quality is a matter of contractual law and everyone should be wary of a specification that quotes ‘defect free’ unless it comes complete with a clear definition of what constitutes a ‘defect’. We all have our own view and, after all, you can reject anything, if you look closely enough. Many trade coaters have long suspected that the degree of acceptance rapidly increases if their customer is suddenly beset by plant breakdowns or strikes, for example.

Most commonly, perhaps, the dispute start when one party in the chain suddenly spots a defect that they find unacceptable, at which point, everyone upstream starts quoting the magic mantra that the coating meets ‘the commercial standard’. Well, that depends on what is meant by the commercial standard. There are some things that I consider inherent: e.g. adhesion, surface hardness and chip resistance are important factors. Even then, the acceptance level for each needs to be spelt out and agreed.

For example, I was asked recently whether good adhesion was an essential factor in a powder coating. Apparently, the coating was becoming badly chipped and falling off in transit and in assembly. That sounded suspiciously like undercuring. The implication from the powder applicator seemed to be that good adhesion had not been specified. Most powder manufacturers would naturally expect their coating to stick to the substrate; otherwise, it could not do its job. The data sheets usually quote well-defined, straightforward standard tests that can be used to test the coating.

It may sound obvious but we naturally expect that any coating would have been selected to meet the rigours of handling, transport and assembly, as well as any other factors associated with its end-use. But we still recommend that appropriate tests are written into any formal agreement on test procedures, to prove that the coating actually reaches the specified standard, confirming that the application process had been carried out correctly.

Another difficulty can arise when the coating problem occurs and there are no established acceptance standards. Then - human nature being what it is - many arguments quickly get out of hand. In our experience, what usually happens is that a coating is applied and everything bumbles along happily until a significant number of components are suddenly rejected. At that point, everyone begins to look very closely at every aspect; so closely in fact that every defect becomes a major debating point.

Very often, it is possible to show that, in hindsight, the problem was always there to a greater or lesser degree. (You know, I like to think that about climate change too, despite Mrs Margaret Beckett’s comment that anyone who disagrees with her views on the subject is no more than a terrorist – but I digress …)

Factors such as colour match, texture, distinctness (DOI) of image and degree of orange peel are much more difficult to define because they are interpretative. For some general fabrications, minor differences in colour from batch-to-batch may not be too important. In fact, in some cases, orange peel is expected, since it can hide a multitude of sins such as spot welds and scores in the surface.

We also need to consider whether some surfaces are more important than others. Will they be seen? Will they be hidden but vulnerable to attack by chemicals or corrosion? We need to define at the outset what we consider to be the ‘A’ faces - i.e. those areas that need to be spot on (or rather ‘spot free’). Often, large areas of the coated surface will be hidden permanently, after assembly.

Automotive exterior body parts – valences, wing mirrors, door handles etc. - are often coated in factories miles away from each other, using different substrates and application methods, yet these days they are all expected to be matched to a single colour on the vehicle. Colour and flow can easily be affected by such factors as film-thickness and substrate grin-through.

It is easy to see that colour, gloss and flow characteristics must be common all over the vehicle and therefore must be closely specified to avoid an effect like a patchwork quilt. There, delta E values of perhaps 0.2 and 0.3 might be specified, also taking into account any metameric effects. On the other hand, a delta E of less than 1.0 is often quoted by coating suppliers as ‘commercial standard’. The automotive industry is also interested in distinctness of image and there are now sophisticated tools for numerically measuring that factor too.

Whatever the application, when specifying colour or gloss, it is important to give an acceptance range and to take special care if parts have to be matched to others on the same assembly.

We have seen otherwise perfectly good applications marred by poor jigging: components ‘shadowing’ each other during electrostatic application; items jigged too close together so that they become damaged when they touch at points where the conveyor bends or changes in height.

The biggest cause of arguments is that of inclusions (i.e. particles or fibres of contaminant, stuck in or on the film.) One excellent method for defining this is to use slide transparencies, printed with markings defining the frequency of inclusions and their size. The problem of sorting out how the contamination got there is quite another story.

Good control of the coating processes is vital, from the initial stages of setting acceptance standards to finally delivery and approval by the customer.

It is essential to agree standards since, even with apparently simple problems, disagreements can arise. British Standard BS EN ISO 4628:2003, BS 3900-H1 to H6:2003 relates to the evaluation of defects that appear during accelerated tests but it may be a useful starting point in developing ideas about size and frequency of defects and of acceptance levels.

Contact us for more details