Dip, Flow & Curtain Coating
Three gravity-driven factory methods that coat parts by immersion, by flooding or by passing them through a falling sheet of liquid — simple, efficient and highly dependent on viscosity control.
Key takeaways
- Dip coating immerses a part in a tank; flow coating floods it with a stream of coating; curtain coating passes a flat part through a falling sheet of liquid.
- All three are high-efficiency methods because excess coating drains back to a reservoir for reuse.
- Film thickness is controlled mainly by viscosity, withdrawal or line speed and drainage time — not by operator technique.
- Typical defects are thick bottom edges, sags, drips (“tears”), trapped air and solvent-related bubbles.
Before spray guns became universal, dipping was one of the main ways to paint manufactured goods, and it remains widely used today for primers on structural parts, agricultural and construction components, fasteners, springs, wire goods, castings and wood products. Flow and curtain coating extend the same principle — let gravity and the coating’s flow behavior form the film — to larger or flatter parts.
These methods should not be confused with electrocoating, which also immerses parts in a tank but uses an electric current to deposit the film, or with coil coating, where flat strip is coated by rollers.
Dip coating
In conventional dip coating, a part is lowered into a tank of coating, held long enough for air to escape and the coating to wet all surfaces, then withdrawn and allowed to drain over the tank before moving to flash-off and cure. Parts may be dipped individually on racks, in baskets, or continuously on a conveyor that dips and lifts each part.
What controls film thickness
- Viscosity and solids. Higher viscosity and higher solids leave a thicker film.
- Withdrawal speed. Faster withdrawal generally carries out more liquid, producing a thicker film; slow, steady withdrawal gives thinner, more uniform films.
- Drain and flash-off. Solvent evaporation during drainage raises viscosity and “freezes” the film; drain time and air movement affect final thickness and leveling.
- Part orientation. Hanging angle determines where coating collects and where it drains freely.
Tank management
Because the bath is reused for a long time, it must be maintained. Solvent evaporates, pigments settle and contaminants accumulate. Tanks typically need agitation or circulation, filtration, temperature control, regular viscosity checks with an efflux cup (for example a Ford cup per ASTM D1200 or an ISO flow cup per ISO 2431) and controlled additions of thinner and fresh material. See rheology and viscosity and thinning coatings correctly.
Flow coating
Flow coating pumps coating through nozzles or a manifold that floods the part as it moves through an enclosed booth. Excess runs off into a collection trough and back to the reservoir. It suits large or awkward parts such as machinery frames, trailer chassis and agricultural equipment that would need an impractically large dip tank.
Flow coaters are often followed by a solvent-vapor tunnel, which slows evaporation so the film can level and drain evenly before it sets. Thickness depends on viscosity, flow rate, part geometry and the time and atmosphere in the drain zone.
Curtain coating
Curtain coating forms a continuous falling sheet of coating from a slot or weir. Flat parts — panels, doors, boards, sheet goods and some furniture components — pass horizontally on a conveyor through the curtain. Excess coating falls into a trough and recirculates.
Film thickness is set mainly by the curtain’s flow rate relative to conveyor speed: a faster conveyor gives a thinner film. The method produces very uniform, smooth films on flat surfaces and is commonly used with UV-curable finishes in wood and panel processing (see UV- and EB-curable coatings). It cannot coat vertical edges or complex shapes well.
Comparing the three methods
| Factor | Dip coating | Flow coating | Curtain coating |
|---|---|---|---|
| Best part shape | Small to medium, any shape | Large, complex assemblies | Flat panels and sheets |
| Coverage of recesses | Excellent inside and out (if air can escape) | Good on external surfaces | Top face only |
| Thickness uniformity | Moderate; thicker at bottom edges | Moderate | Very good on flat faces |
| Material volume in system | Large tank inventory | Smaller reservoir | Smaller reservoir |
| Color change | Slow and costly | Moderate | Moderate |
Advantages and limitations
Advantages
- High material utilization with little overspray
- Simple equipment and low labor per part
- Dip coating reaches inside tubes and hidden surfaces
- Consistent results once bath and line parameters are set
Limitations
- Uneven thickness: fat edges, sags and drips at low points
- Large open tanks raise solvent emission and fire concerns
- Bath stability and contamination need constant control
- Limited choice of fast-curing or two-component coatings due to pot life
Large solvent-borne dip and flow systems are significant fire and emission sources. They require proper ventilation, fire protection and compliance with local air quality rules; see VOC regulations. Many operations have switched to waterborne dip primers for this reason.
Common defects and fixes
- Fat edges and drips: reduce viscosity, slow withdrawal, re-orient the part so it drains to a single point, or use electrostatic “de-tearing” where available.
- Thin films: viscosity too low, excessive thinning, or slow withdrawal combined with fast solvent loss.
- Air pockets and bare spots: trapped air in cavities; re-hang parts or add drain and vent holes.
- Bubbles and solvent pops: too thick a film, too short a flash-off or too fast a cure ramp.
Log bath viscosity, temperature and additions at a fixed time each shift. Trends in that log usually predict quality drift before rejected parts appear.
Frequently asked questions
Is dip coating the same as e-coating?
No. Both use immersion, but e-coating uses electric current to deposit a controlled, uniform film. Conventional dipping relies on viscosity and drainage.
How thick is a typical dip-coated film?
It varies widely with the coating and process, but single-dip industrial primers are commonly around 1 mil (25 µm) or a little more per coat. Check the product data sheet.
Why are my parts thicker at the bottom?
Coating drains downward and collects at the lowest edge before it sets. Adjust viscosity, withdrawal speed, hanging orientation and drain time.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
