PVA release agent can be applied evenly to a mould using a spray gun. Proper application is important: once dry, the PVA must form a continuous and even film between the mould and the part. The most common mistake when spraying PVA is applying too much material straight away. This can cause the PVA to run, build up or form an uneven film.
Work in a controlled manner. Start with a light mist coat and then build up the PVA film in several passes. This gives you more control over the coverage and reduces the risk of runs and other irregularities.
Why is spraying a good way to apply PVA?
Once dry, PVA forms a physical release film on the surface of the mould. For reliable demoulding, this film must completely cover the mould surface. A spray gun allows you to atomise PVA finely and distribute it evenly over larger and curved surfaces. This makes spraying particularly suitable when the quality of the PVA film is important.
Another advantage of spraying PVA is that you can build up the amount of material gradually. You do not have to apply a complete wet coat in a single pass.
Which spray gun should you use for PVA?
PVA can be applied using different types of spray guns. A simple conventional siphon-cup spray gun, for example, is very suitable.
There is no universal combination of nozzle size and spray pressure that works for every spray gun and every PVA release agent. The correct settings depend on factors including:
- the type of spray gun;
- the nozzle;
- the viscosity of the PVA release agent;
- the air pressure;
- the material flow;
- the distance from the mould;
- temperature and ambient conditions.
Start with a low material flow and adjust the spray gun to produce a fine, even mist. Preferably test the settings on a smooth test surface before spraying the mould.
| Setting | Guideline |
|---|---|
| Spray gun | Conventional siphon-cup |
| Material flow | Start low and gradually increase |
| Atomisation | Fine, even mist |
| Nozzle | 1.0-1.4 mm |
| Spray pressure | 1.5-2.0 bar at the spray gun |
| Spray distance | Approx. 15-25 cm |
| Final result | Continuous and even PVA film |
Use these values as a starting point. The quality of the final film is more important than one fixed spray gun setting. You want a fine, even mist without making the first coat wet.
How to spray PVA in 5 steps
Follow the steps below to spray polyvinyl alcohol:
- Prepare the mould
Make sure the mould surface is clean, dry and free from dust, grease and other contaminants. Are you using PVA in combination with release wax? Apply the release wax first according to the instructions for the release system you are using.
Before spraying, also check whether the PVA spreads evenly over the prepared surface. If the liquid immediately pulls back strongly or is repelled in certain areas, a continuous film cannot form. - Start with a light mist coat (dust coat)
Set the material flow low and make sure the PVA is finely atomised. Then apply a very light first pass. The polyvinyl alcohol does not need to make the surface wet during the first pass.
The purpose of this first coat is not to create the complete final PVA film immediately. Instead, you first create a thin base on which you can build up the following passes in a controlled manner. Keep the spray gun moving continuously while spraying and try to maintain a constant distance from the surface.
Then allow the first coat to set according to the instructions for the PVA you are using. - Continue building up the PVA film
Apply additional passes and gradually increase the material flow if necessary. You can alternate the spraying direction between passes. For example, spray horizontally first and then vertically. This makes it easier to cover the entire surface evenly.
Do not try to achieve the desired film with one very wet coat. Building up the film in a controlled manner gives you more control over the final result and reduces the risk of runs and local build-up. - Check that the film is completely continuous
Inspect the entire mould surface after application. The PVA must form a continuous film everywhere. Pay particular attention to pinholes or small holes, dry or barely covered areas, areas where the PVA pulls back, runs, local build-up and differences in coverage.
A useful method is to spray a small, smooth test panel alongside the mould. This allows you to check the build-up and drying of the PVA without touching the mould surface. - Allow the PVA to dry completely
Allow the applied PVA to dry completely before applying production gelcoat or resin. The required drying time depends on the product used, the amount applied, temperature, humidity and ventilation. Therefore, do not use one fixed drying time for every situation.
Only proceed to the next step once the PVA film is completely dry.
How thick should you spray PVA?
When spraying PVA, a completely continuous film is more important than a universal film thickness. The required film thickness can vary depending on the PVA product and application method. Therefore, use the technical data for the PVA you are using if a specific wet or dry film thickness is specified.
In practice, you should mainly focus on the result: once dry, the PVA should form an uninterrupted film without pinholes, dry areas, runs or other visible gaps. Applying PVA thinly does not mean that one barely visible mist coat is sufficient. The aim is to build up the film in a controlled manner until the surface is completely and evenly covered.
How do you recognise a good PVA film?
- The film is completely continuous
There are no pinholes, dry areas or other gaps that could allow the production gelcoat to come into direct contact with the mould. - The film is even
There are no major differences in film thickness, runs or local build-up. - The PVA does not pull back
If the PVA is locally repelled and collects in droplets or islands, a reliable continuous film will not form. In this case, check the mould surface and the release system being used before continuing. - The film is completely dry
Do not apply gelcoat or resin while the PVA film is still wet.
Why is a PVA film that is too thin a problem?
PVA works by forming a physical release film between the mould and the part. If this film is missing in certain areas, the physical barrier is also absent in those areas.
A very thin or interrupted film can therefore provide a false sense of security. You may have applied PVA, but the surface is not actually protected everywhere by a continuous film. Do not try to spray as little PVA as possible. Apply it in a controlled manner until a continuous, even film has formed.
Common mistakes when spraying PVA
| Mistake | What happens? | What can you do? |
|---|---|---|
| Spraying too wet from the start | The PVA can run or build up locally. | Start with a light mist coat and build up the film gradually. |
| Applying too much material in one pass | The film can become uneven and develop runs. | Work with several controlled passes. |
| Applying too little PVA | Pinholes and dry areas may remain in the film. | Continue building up the film until it is completely continuous. |
| Incorrect spray gun settings | The PVA is not atomised sufficiently or excessive overspray occurs. | Adjust the air and material flow for fine, even atomisation and test the settings beforehand. |
| Contaminated mould surface | The PVA may be repelled locally or spread unevenly. | Clean and inspect the mould surface before applying PVA. |
| Insufficient drying time | The PVA has not yet formed a completely dry release film. | Allow the PVA to dry completely before applying gelcoat or resin. |
Spraying PVA on a new polyester mould
PVA is often used when conditioning a new polyester mould. The PVA film then forms an additional physical barrier between the new mould and the first part.
At Polyestershoppen, we recommend producing the first two parts in a new polyester mould using PVA. After that, you can generally switch to the normal release system.
Want to know more about PVA or making a polyester mould?
When spraying PVA, it ultimately does not come down to one exact nozzle size, spray pressure or fixed number of passes. Spray guns, PVA products and conditions vary too much for that. Instead, focus on the result. A good PVA film is continuous, even and completely dry before you apply gelcoat or resin.
Do you still have questions about spraying PVA, adjusting your spray gun or preparing a polyester mould? Please contact our specialists! We will be happy to help.
