Liquid vs Solid Paint Content – Why Does It Matter?

Traditionally the industry has talked about paint in terms of litres, but more and more are coming to realise that this is an unhelpful method, and inaccurate by almost any measure.

The key to understanding this is that all paint is made up of two main parts – liquid and solid. Of these, the liquid will evaporate during drying/curing, leaving only the solid part remaining to form your painted surface. This means that many aspects of coating performance are dictated by the solid content – opacity, coverage, dry film thickness etc.

The liquid part also affects the performance, but more in terms of application and finish. It determines how well the product flows out, which influences adhesion and the surface appearance but cannot add to what is physically left on the substrate once cured.

Comparing Products

What this means for you is that comparing paint products by a volume measure such as litres is certainly not a reliable method of determining the actual value to you. As an example:

• Product A comes in a 5L can, costs £6 per litre and contains 50% solids. You are therefore paying £30 for 2.5L of solids - £12 per litre.

• Product B (also in a 5L can) costs £7.50 per litre, apparently 25% more expensive than Product A. However, it contains 70% solids which is 3.5L – so at £37.50 for the can, you are paying £10.71 per litre, more than 10% saving!

This is not “blinding with science” or any attempt at pulling the wool over people’s eyes, it is the straight facts of the physical nature of paint coatings.

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Litres or Kilograms (KG)?

The confusing part comes when we start to talk about paint in terms of kilograms (KG), as it is increasingly being sold. It is true that this is normally a much more accurate measurement for calculation/comparison – this is because paint can have a widely varying weight-to-volume ratio. This is often expressed as “Specific Gravity” or “Relative Density” – typically between 1 and 1.4 kg per litre. The ratio is most affected by the proportion of solid content – for 2 cans weighing 5kg, one can might contain 5L where the other is only 4L, but it is fairly safe to assume the useful solid part is roughly equivalent which should be the basis of your value assessment for coatings.

However, this is only safe to assume this when comparing similar types of paint, where the chemical make-up of the solid element is also likely to be similar, meaning the weight-to-volume ratio is comparable. This can vary a lot between different types of paint, for example, a paint that has a high zinc content will have a very high ratio as zinc powder is very heavy for its volume, but the solid element of another paint that is designed to do the same anti-corrosive job in a different way may have a much lower ratio but still achieve a similar or better dry film thickness and protection.

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Be Aware – Solid Content Matters!

Overall, the important thing is to be aware of the danger and inaccuracy of comparing or measuring paints by volume (litres) only – look at the data sheets and compare the volume solids % figures, and the Specific Gravity or Relative Density values, and talk to your paint supply partners about what they mean in your specific circumstances.

Normally when considering paint options it is possible and sensible to carry out practical applications with wet and dry film thickness and/or weight measurements to achieve an accurate cost-per-square-metre applied comparison.

We have provided a sheet with some of the common formulas you can use to convert the values discussed above into coverage and usage rates etc and will be addressing the topic of film thickness and weight in later articles.