Blooming, also called blushing, is one of the most common calls we get in autumn. A sprayer puts a clear coat on in the morning, comes back an hour later and the panel has a white, milky haze. The lacquer is fine. The air in the booth was wet.
What is happening in the film
As solvent evaporates from a wet coat, it cools the surface of the film. Fast-flashing solvents cool it more. On a dry day this makes no difference. On a humid Irish morning, the film can cool below the dew point of the surrounding air, and water vapour condenses straight into the wet lacquer.
That water is trapped as the film dries. It scatters the light, and you see a white or grey haze, most obvious on dark colours and clear coats over dark timber.
Conditions that make it more likely:
- High humidity: wet mornings, rain blowing into an open roller door, a booth next to a wash bay.
- A cold workshop, or cold stock and cold panels brought in from a van or a yard.
- Fast thinners, or spraying a thin coat that flashes quickly.
- Spraying late in the day as the temperature drops.
Prevent it with temperature first
The most reliable fix is to keep everything warm: the air, the product and the work.
- Warm the booth before you start, not when the first panel has bloomed.
- Bring stock in the night before. A tin that spent the night in a van is at van temperature, and it will cool the film further.
- Let panels reach booth temperature. Doors carried in from a cold store need time before they are sprayed.
- Keep doors closed on wet days and do not spray next to anything producing steam.
The general rule in coatings is to keep the surface at least a few degrees above the dew point. A simple hygrometer in the booth tells you the humidity and temperature, and on bad days it tells you when to stop.
Then slow the thinner
A slower thinner evaporates more gently, so the film cools less and stays open long enough for any moisture to escape.
- Novalk acid-cured and pre-catalysed. Blend Anti Bloom Haze Thinner ND4800 with Premium Thinners ND4100. The Novalk PC clear topcoat sheet gives half ND4100 and half ND4800 as the anti-blooming mix, and the PC sanding sealer sheet lists ND4800 for anti-blooming too.
- Sirca polyurethane. Use DPU805 retardant thinner with the normal PU thinner.
Keep to the total thinning percentage on the product sheet. You are changing the speed of the thinner, not the amount.
If a panel has already bloomed
- While it is still soft, moving the panel somewhere warm and dry can let light bloom clear as the trapped moisture escapes.
- Some sprayers mist a light coat of the same lacquer thinned with the slow thinner over light bloom, to reopen the surface and let it release. Test this on one panel before doing a batch.
- If the haze has cured into the film, it will not polish out. Let it harden fully, flatten the surface and recoat in better conditions with an anti-bloom thinner.
Do not keep adding coats over bloom hoping it will go away. Each coat traps more of the problem.
As the weather turns colder, blooming is joined by slow curing and soft films; see spraying in a cold workshop. In summer, the opposite problem appears: spraying lacquer in hot weather covers dry spray and short pot life.
Waterborne is affected too, differently
Waterborne lacquer does not bloom the way solvent lacquer does, but cold, damp air slows its drying a great deal and can stop the film forming properly. If you are moving to waterborne, plan for heating and airflow. Our guide to switching to waterborne lacquer covers the workshop side.
Order the slow thinner before you need it
Anti-bloom thinner is the kind of product that is always needed on the one morning you do not have it. Add a tin to your next order, or build a quote with it alongside the lacquer. For more on keeping stock in condition through the colder months, read how to store lacquer and paint in an Irish workshop.