National Physical Laboratory

Use of Paints and Coatings

Painting or coating is the mainstay of corrosion prevention for material not inherently corrosion resistant. This is a matter of economics. Paints and coatings are generally applied at low additional cost to the least expensive structural materials. Minimising the additional cost is an integral part of ensuring the economic viability of painting and coating as a corrosion control solution. Therefore paints and coatings generally do not last as long as the operating lifetime of the material to be protected, the ability to maintain the coating systems is vital.

material costs

Chart depicting materials costs as a small proportion of overall costs


Factors in selection of paint or coating systems

When choosing a system, several alternatives may appear to be technically acceptable, and it is necessary to identify relevant factors affecting corrosion control and cost.

The most important of these are:

  • High project cost, prestige or failure consequences may warrant choice of high performance materials
  • Type of substrate to be coated
  • Track record of the selected system for the environmental and operating conditions expected
  • Life expectancy of coating to first maintenance
  • Ease of access to substrate work surface
  • Quality of applicator and contractor
  • Compliance with legislative and environmental requirements
  • Delivery logistics
  • Maintenance conditions, and compatibility with existing materials
Protection duration needed Requirement for coating type
Short term
(1-5 years)
Single pack materials such as alkyds, emulsions and acrylics are usually in this category
Medium term
(5-10 years)
Two-component materials such as zinc-rich primers, epoxies, polyurethanes, usually at lower thickness
Long term/High performance
(10+ years)

Two-component materials such as metal spray, epoxies, polyurethanes, glass flake and FRP  


Coating types and their lives

See the following brochures for further information:

Costing of a single system
The costing of a single system must be a compromise between various individual cost factors, typically including:

  • Materials
  • Labour
  • Transport

Cost of comparisons of different coating systems
These are more complex, and such aspects as painting conditions, substrate preparations, humidity, temperature and time available must all be considered.

 

Estimation of painting costs
Painting costs are estimated either as total cost or cost per square metre. Several objective and subjective factors are important for estimating valid painting costs.

Overspray of fabrication
Even on uncomplicated substrates this can amount to 5% despite efficient airless spraying.

Wind loss
Paint application in the open high wind can cause losses of 20% to 50%.

General losses
Factors such as an exceeding pot life, pilferage and spillage need consideration in estimating paint consumption.

Absorbent surfaces
A factor of 2% to 5% should be considered for inadequately sealed wood surfaces, as they tend to absord paint into the substrate.

Paints and coatings

Peripheral factors affecting the choice of the supplier

Coverages rate and paint types

Solventless and solvent-free coatings are invariably much more expensive than normal solvent-bound types but, for a given DFT, they yield a much higher coverage rate than solvent-bound types. The example below illustrates this for the three coating types at a coverage of 200 micron DFT. In this case, the use of solvent-free materials (paint 2), though more expensive in cost per litre, actually work out cheaper.

Example calculation of paint costs

Note: DFT equals dry film thickness, WFT equals wet film thickness

Noting that, by definition, 100 x DFT/WFT = VS%

And then consequently, that WFT = DFT x 100/VS%, then for:

Solventless paint 1:

VS% = 97.5% at £4.80 per litre

Solvent-free paint 2:

VS% = 100.0% at £4.90 per litre

Solvent-bound paint 3:

VS% = 50.0% at £2.70 per litre

Paint 1 WFT = 250 x 100/97.50 = 256.41 micron

Paint 2 WFT = 250 x 100/100 = 250 micron

Paint 3 WFT = 250 x 100/50 = 500 micron

Accordling, the paint volumes to cover 1 square metre are (litres):

Paint 1 = 256 x 100 x 100/10.000ml = 0.256 litres

Paint 2 = 250 x 100 x 100/10.000ml = 0.250 litres

Paint 3 = 500 x 100 x 100/10.000ml = 0.500 litres

And the costs are:

Cost/m2 for Paint 1 = £4.80 x 0.256 = £1.23

Cost/m2 for Paint 2 = £4.90 x 0.250 = £1.22

Cost/m2 for Paint 3 = £2.70 x 0.500 = £1.35



 

Last Updated: 19 May 2014
Created: 30 Nov 2007

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