Overspray cratering - small round indentations (normally evenly spread over the affected area). Large craters may have slightly raised-ridges.

Possible causes:

Incompatible spray dust on or falling into wet film from previous operation, under balanced spraybooths can cause overspray to hang around for a long time, overspray disrupted from operators overalls or equipment can occur.

Shear – spotty, non-uniform blotchy appearance of metallic paint, or small darker irregular areas in solid colours.

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Possible causes:

Normally associated with excessive wet films in straight colours and single  layer metallics, high viscosity, slow evaporation of thinner, low atomizing pressure, narrow fan width, gun too close, failure to overlap by 50% gun strokes, defective spray pattern or failure to hold gun at right angles to work.

In basecoats, shear may be caused by the above, but may also be produced because paint is applied to dry and is not able to coalesce (combine to form L a uniform film) resulting in small areas of dry spray resembling leopard spots.

Seed - the appearance of uniform particles within the paint which have a regular size and pattern.

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When seed is formed in paint, it is the action of particles coming together, either pigment or resin and will give a regular pattern. It can happen with thin coats of paint where coarse pigment can often be seen protruding through the film and is more likely to occur with old batches of paint. Paint filtration should remove seeded paint.

Floating/Flocculation - the tendency of pigment particles in wet paint films to separate from one another and concentrate in particular areas.

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Floating defects are the result of applying heavy wet films of paint. To avoid the application of very wet films in metallic finishes, particular care must be paid to paint technique, paint viscosity and atomizing air pressure. In brushing, particular care should be taken around screw heads, etc. to avoid the development of runs, which may show float lines.

Flooding defects are more common in brush application and are usually the result of 'working‘ the paint by brush after the initial set up has occurred. In spray finishing, excess air pressure can cause colour difference.

Sand scratches (scratch swelling) - the appearance of imperfections showing from the primer or petal.

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Likely causes include low build of topcoat from over thinning or insufficient coats, use of too coarse flatting paper, inadequate drying of first colour coat or poor holdout of primer. Lacquer dry primers have poorer holdout than two packs due to solvent swell. but under cured two pack primers will also solvent swell, showing scratches and sinkage.

Poor hiding power (opacity) - the inability of a paint to mask the colour or pattern of a surface.

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Most paints are designed to obliterate the under surface with a Specified number of coats, some very clean yellows, reds or green may have poor opacity due to the nature of the pigment used. Likely causes for poor opacity of paints not in the above category are the use of insufficient number of coats, over thinned paint, use of paint not fully stirred (goodness remaining at the bottom of the tin) or of applying paint onto a bleeding pigment or dye in the colour beneath.

APPLICATION FAULTS COMMON IN THE PAINTSHOP

Gloss Level

All colour coats have their pigments dispersed to 10 microns or finer and are formulated at a pigment: binder ratio that gives the highest gloss consistent with adequate opacity.

Full gloss and opacity are achieved at a dry film thickness of 50 microns (0.002“) in 3 single or 1 single and 1 double coat for the majority of colours, 2 single in MS systems.

Therefore, lack of gloss is usually due to application conditions and technique.

The above causes can occur under normal conditions of 65-70oF (18-21⁰C) and would become more pronounced at higher temperatures of 75-80oF (24-27⁰C).

         Permobel                                                Very rarely occurs

         Permobel                                                Not likely to occur.

         Permobel                                                Not likely to occur.

         Permobel                                    In the unlikely event, treat as above.

        Belco, 2K                                                Unlikely to occur.

        Permobel

        P383.

A good description of problems, together with coloured photographs is given in the Refinishers Handbook which is available to all staff. Because the information is well covered in this book I will not reproduce it in this module, but refer the student to its use for reference.

LOOKING AT CURRENT TRENDS

The average car has an accident every 50,000 miles, costing about £700 to repair. 65% of all damage-is to the front, with QOX being 0/8 front wing, bonnet and front panel.

While cars become more reliable and more intricate with fewer manufacturers producing larger volumes, the customer requirements for increased specifications including durability and corrosion resistance has led to the increased use of plastics, alloys and galvanised protection. Repairing of these materials will have their own problems, but demand for paint will still remain.

WHAT MAY WE EXPECT FROM THE FUTURE?

There are two main areas which will demand change. 1) The requirement by advertisers and marketing to sell something different about their car; different colours, special effects, etc and who knows what that will bring. 2) the ecological pressures to produce more environmentally friendly products and we have already seen rule 1151 come into force in Southern California, which requires the reduction in volatile organic compounds (VOCs) to 3.5 lbs/gallon in refinish shops by January 1992 and our own EPA regulations.

So the likely trends are reduction in organic solvents by moving to higher solids, using non organic solvents (water) or doing away with solvents all together and using powder. But why should we be reducing the use of volatile organic solvents?

After the Earth formed and cooled, the atmosphere was thought to have contained no oxygen and so was bathed in high doses of UV radiation from the sun. It is thought that life forms first formed more than 10 meters below the surface of some stagnant pool of water. 10 meters of water being necessary to filter out the UV radiation which would destroy cells by breaking chemical bonds. As oxygen levels increased, so Ozone 0₃ was able to form in the upper atmosphere, which absorbed UV radiation and allowed life forms to rise out of the protective water. The level of gasses is finely balanced. It is thought that the Ice Ages are related to times of over production of oxygen and a reduction of carbon dioxide by processes on Earth and the recent increase in Carbon Dioxide may lead to a warming of the Earth.

With this background, what are the reasons for controlling volatile organic compounds within the Paint business as covered by the Environmental Protection Act? Most of the known work follows problems of pollutants emitted into the atmosphere, causing smog in Los Angeles and the understanding of the causes and effects. It should be mentioned that all major cities suffer the same pollutant effects, especially with high motor car usage and that more people suffered medical problems from London smog from the burning of large amounts of smoke coal before the Clean Air Act and the use of smokeless fuels.

There are several factors that lead to the production of photochemical smog. The 1ocal topography in Los Angeles often results in lighter warmer air lying on top of colder heavier air. A very stable situation known as 'temperature inversion' with very little air movement, leading to the trapping of pollutants at or near ground level. Intensley sunny days are frequent and there is a high density of car usage. What is observed on smoggy days is that there is an increase in Ozone O and Nitrogen Dioxide No; towards noon with a brown haze which irritates the eyes and respiratory system and is known to restrict plant growth. Under severe conditions, exercise is advised against and those with respiratory disorders are kept indoors. What is known is that these primary pollutants react together with sunlight and the unburnt hydrocarbons, mainly unburnt exhaust petrol, but paint solvents and even fine oil released from pine trees, to form secondary pollutants.

The most common source of air pollution resulting from human activity appears to be the combustion of fossil fuels. The relatively high temperature of the internal combustion engine converts some atmospheric nitrogen and oxygen into NO (mainly NO) Nitrous Oxide, Carbon Monoxide (CO) and unburnt hydrocarbons. Primary pollutants with air then undergoes chemmical change on irradiation with sunlight with NO being oxidised to NO₂ by catalysic and the presence of hydrocarbons leading to the formation of Ozone O . The formation of Ozone in the upper atmosphere is essential for man, but the formation at low levels leads to health hazards and Peroxyacetyl-Nitrate, PAN, being the main irritants to respiratory systems and eyes.

The problem of all atmospheric pollution has been likened to a bath with the taps turned on and the plug out. Over the last 4-6 thousand million years an equilibrium has built up by some 50 to 60 chemical reactions known to occur in the atmosphere. Over the last 200 years we have been pumping out all kinds of material into the atmosphere and we know very little of what effects it may have on the future. We don't know if we have a small hole in the bath or whether we have kicked the side in.

One thing is for sure, if we go on expanding our population at its present rate, consuming raw materials at an ever increasing rate and polluting our planet at an ever increasing rate the dinosaurs will have ruled the planet longer than us.