The goal in applying a paint coating is to provide a film which will give protection and decoration to the surface being painted. The success of any paint application will be governed by a number of parameters, including, surface preparation, film thickness, climatic conditions prior to, during and
after application, and methods of application.
The importance of surface preparation to the success of a paint system cannot be overemphasized. A separate section on surface preparation has been included in this manual (Section 6). The required paper work for paint work is provided in Attachments B, C, D, and E of this manual.
PAINT APPLICATION GUIDELINES
An adequate film thickness is essential for the success of any coating system. Low thickness application will generally result in premature failure for obvious reasons. However, the old adage of “the more paint, the better” can be equally dangerous. The gross over application of modern high technology paint coatings can lead either to solvent entrapment and subsequent loss of adhesion, or to splitting of primer coats. With the majority of coatings, the limits of acceptable dry film thickness allow for reasonable practical variation, but the correct film thickness should always be
the target during application.
The actual dry film thickness recommended for a particular surface will depend on the type of paint system being used and the nature of the surface. Recommended dry film thicknesses for individual products are given on the Product Data Sheets and System Specification Sheets in this manual.
When applying paints, the most important factors to consider are the state of the surface, the surface temperature and the atmospheric conditions at the time of painting. During the night, steel temperatures fall. They rise again during the day, but there is always a lag in movement of steel
temperature compared to the atmospheric condition, so condensation on the steel surfaces is possible.
Climatic condition is generally of importance in the application protective coatings especially in coastal environments. During coating application, relative humidity should not be more than 90% and condensation should not be allowed to take place on the surface being painted. In order to
determine whether or not a surface is wet, the steel temperature should be measured using a surface temperature thermometer and the dew point calculated after measurement of humidity with a hygrometer. Paint application should not take place when steel temperature is less than 3°C (5°F)
above the dew point. Paints should not be applied when surfaces are affected by rain or ice. Some two pack paints (certain epoxies for example) should not be applied at low temperatures as curing may be retarded.
Generally, extreme conditions refer to temperatures below 5°C (41°F) or above 35°C (95°F). Below 5°C (41°F), the curing of some paints such as epoxies will slow down dramatically and for some paints, curing stops altogether. Chlorinated rubbers (specified in this manual for road markings) and vinyls are quite suitable for use at temperatures below 0°C (32°F) provided that the surface is clean and free from ice or frost.
At the other extreme, 35°C (95°F) and above, the drying and curing of paints is rather rapid and care should be taken to avoid dry spray. This is caused by the too rapid loss of solvent from paint droplets between the spray nozzle and the surface. It can be overcome/avoided by:
1. Keeping the spray gun at the minimum suitable distance from the work piece, spraying consistently at 90° to the surface being painted.
2. Adding thinners, if necessary, as recommended by manufacturer.
Methods of Application
An airless spray gun atomizes a paint stream by ejecting it at a high pressure from a specially designed tip.
The area sprayed should be within a comfortable distance of the operator to avoid a long traverse of the jet or arcing of the gun.
It is essential that due regard be paid to safety because an airless spray gun ejects a paint stream under very high pressure, and injury can be caused if the jet is directed at someone close by. The skin can be easily penetrated.
A conventional spray gun forms paint droplets by mixing air with the paint stream but turbulence will be rather higher than with airless spray. To ensure the paint is fully dispersed in fine droplets, its viscosity must be low. Conventional, decorative materials and water based zinc silicates are the most common conventionally sprayed coatings.
Brush Application. This method is relatively slow but is generally used for decorative paints or for coating small areas. It is particularly suitable for coating complex and complicated areas where the use of spray application cannot be justified. With most high build coatings it is not possible to achieve the desired film thickness in the same number of coats as for example if the application was done by airless spray. Multi-coat applications may therefore be necessary to give the specific film build.
Roller Application. This method is faster than brush on large even surfaces and can be used for the application of most decorative paints. Control of film thickness is not easy to achieve however, and the same constraints as brush application generally apply. Particular care must be taken, by selection of the correct roller pile length, when coating rough or irregular surfaces.