What is the best way to dry water-based wood coatings

By Bradley Hext

What is the best way to dry water-based coatings in the wood industry?

Wood coatings are used to strengthen and protect wooden substrates from damage caused by sunlight, grease, dust and chemicals. There are a number of different coating types available and water-based coatings, as the name suggests, use water as the carrier into which the chemical components are dispersed. After application to the substrate by spraying, this water is released from the coating through the process of evaporation, until it becomes dry.

Benefits of water-based Coatings

Water-based coatings generally make for a much safer and more pleasant working environment for the sprayer during application and drying. As water-based coatings don’t use solvents as the carrier, there is less smell and associated risks.

Aside from this, the coating releases much lower VOC into the atmosphere when compared with solvent based products, and has essentially zero smell once dry – this makes overnight installation possible for domestic end users, where a traditional coating would continue to emit odours for up to 2 weeks or more.

The history of water-based coatings

Modern R&D has enabled the chemistry behind industrial coatings to improve significantly. One of the first commercially available paint products was a primer based on fish oil! Oil-based products did have their own specific benefits but gave very slow drying times in return. 

 These early technologies were replaced with synthetic alternatives once the distillation processes were available to produce solvents of usable types we’re familiar with today.

 Over the last 20 years, in particular, the technology of water-based coatings has improved to the point where they are now completely suitable for the majority of industrial wood finishing requirements. Their performance once on the substrate is now equal to solvent-based products – they pass the same industry recognised tests for durability, while emitting around 10x less VOC’s during application.

The downsides to using water-based coatings on wood

The relevant spray equipment to apply water-based coatings is widely available and used, but the single most common downside from an operator’s perspective is the speed of drying vs solvent-based products such as acid-catalysed & precat/cellulose.

 Solvent-based products become surface-dry much quicker than water-based ones because the evaporation speed of alcohol or naphtha-based ingredients is faster than water due to their lower boiling point.

However, while this means that solvent-based coatings quickly become dust-free – they won’t necessarily be dry enough to handle/sand/package – and this is where water-based coatings can catch up given the right drying environment.

There are two stages to the drying of modern acrylic coatings. Firstly, water and small quantities of coalescent solvent are removed, at this point they become dry to touch. After this, the coating cross-links chemically and achieves its durability and protective functions. Before this point the coating will be susceptible to marking from moisture such as rain-spotting on external joinery. 

What factors affect the drying time of water-based coating?

Three main factors play a part in the drying time of water-based coatings: 

  • Film Thickness
  • Temperature (& humidity)
  • Air Movement

 Paint storage is also important, it should not be stored below 10 degrees celsius to maintain its properties & ‘be sprayable’ when required.

Film Thickness:

The higher the Wet-Film-Thickness (WFT) of material applied per coat, the longer it takes for the water contained in the coating to reach the surface, before it can be removed leaving a dry film.

A minute volume of glycols will remain in the film longer than the water, which is the duration until the product reaches full ‘cure’/ durability.

 If a coating is dried at specification weight at ‘room temperature’ – the industry standard phrase often taken out of context (20-23Deg C in lab conditions) – it will often be an acceptably brief process, perhaps 2 hours per coat of paint.

If an unnecessarily high weight is applied, (easily possible with Thixotropic coatings) then the drying time is increased far in excess of this increase in weight – perhaps double.


The issue arises with many UK manufacturers in the wood industry where winter drying is an issue. Many bespoke makers are smaller companies which may not be aware – or have not yet invested in – the readily available methods of maintaining a drying process during colder weather.

Water-based coatings essentially have to remain above a minimum of 15 degrees celsius before, during and after the application process in order to perform correctly. This moderate temperature enables the coating to flow-out properly before forming a film, which improves both the aesthetics the durability. 

A general rule-of-thumb is that drying times nearly half when the temperature is doubled. Get close to 5 degrees celsius and the process essentially stops, go much over 40 degrees celsius and there is a risk of the timber substrate being affected.

So the ideal temperature is 20-30 degrees celsius with a relative humidity of 50-70%.

 Air Movement:

There are 2 factors to consider with the airflow when drying water-based coatings; 

circulation & air-change:

  1. Circulation is fairly self-explanatory: it’s the amount of airflow around the substrate, reusing the existing air within the spray booth/drying room. A velocity inside the drying area of 1.0 – 1.5mtrs/second is useful, (approximately double that of a spray booth)
  2. Air-change is also important, in that it replaces the ‘damp’ air inside the spray booth/drying room with ‘dry’ air which can more readily absorb moisture from the coating. Typically we look to complete 1-2 air changes per hour, calculated by the air volume extracted.
How can the drying time of water-based coatings be improved?

There are three main ways to improve drying time: maintain the film thicknesses within the manufacturer’s datasheet guidelines, increase the ambient temperature to speed up the evaporation process, and increase the airflow around the drying items. 

Sounds simple enough but while the sprayer can easily be taught to apply the coating correctly, affecting temperature and airflow is not so simple without the use of specific equipment.

What equipment can be used to speed up the drying time of water-based coatings?

At Elmbridge, we stock a range of equipment to assist wood-working professionals speed up their drying times. Our range offers something for varying budgets and space requirements.

Graph: Drying time vs Temperature

Flash-off room/area

A flash-off room or area is a dedicated space where the coated items can be left to flash-off before further drying. This would be at ambient temperature with low air-flow & around 70% relative humidity, the coating can find its level before forming a film during the actual drying process.

In a purpose-built spray facility this would normally be an enclosed room to prevent dust reaching the items while still wet, separate to the spray shop to prevent contamination from overspray & the excessive airflow, and often used as a connecting space to the drying room/tunnel.

Infrared heaters

Infrared systems transfer heat to the coated items. They utilise the electromagnetic spectrum to emit infrared energy which is transformed into heat once it reaches a surface – meaning surrounding temperatures can be kept low. IR does not heat air, only mass, hence is very efficient in areas with extraction as the heat created is not taken from the room.

There are 2 main types of infrared heaters, electric or gas-catalytic. 

  • Electric IR produces short-medium wave heat which has relatively little power to penetrate into a coating, however it appears effective because it warms the visible exterior of the product.
  • Gas IR produces medium-long wave heat which travels through any coating system until it hits the substrate beneath. This has the effect of drying the total thickness of the coating simultaneously, rather than requiring the gradual transfer of heat from the outside-in.

Gas IR has the added benefit of being Atex-approved. Due to the chemical reaction, the hot element remains void of oxygen that would be required to combust flammable liquids. 

So as the fastest & safest method, Gas IR is widely accepted across the industry as being the best way to dry water-based coatings.

Schubox® catalytic IR Drying Rooms / Tunnels

The Schubox® is a trademarked name of a particularly refined Gas IR drying system, which has become one of the most widely used drying systems around the world. 

It works at relatively low temperatures (around 30-40 degrees celsius) with PLC control monitoring, so will not affect the structure of a wooden substrate from moisture being removed internally.

Schubox drying rooms are bespoke to suit each customer’s requirements, but typically consist of hinged double-doors both ends, 2-4 x 10kw wall-mounted infrared emitters, forced air recirculation, with bleed to atmosphere to prevent any gradual build-up of VOC.

Typical dimensions range upwards from 3metre (w) x 5m (d) x 2.5m (h), constructed from mirror-polished stainless steel internals to give optimum reflection inside, with insulated wall construction to reduce energy consumption. 

What method do you use to dry water-based coatings ? If you’re still unsure whatis best for you or want more information before making an investment then don’t hesitate to contact our sales team on 01452 525757, or fill out the form below and we can discuss your specific requirements.

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