Fundamentals of thinning ratio, solids content, and film thickness

On coating shop floors, paint is thinned with thinner or water to make it easier to apply.

The paint is too thick.
It is difficult to spray.
The finish is rough.
It sags easily.
It is difficult to apply.

In such cases, operators may add thinner while checking viscosity to adjust the paint.

Viscosity control is important in coating operations.
However, there is a point that requires attention.

Even when viscosity is correct, coating film quality is not necessarily stable.

This is because coating quality is not determined by viscosity alone.

To stabilize coating quality, thinning ratio, solids content, thinner type, wet film thickness, dry film thickness, and drying conditions must be considered together with viscosity.

Viscosity is an important indicator of ease of application

Viscosity is an indicator of how easily paint flows.

If viscosity is too high, the paint becomes thick and difficult to spray.
The finish may become rough, or leveling may deteriorate.

Conversely, if viscosity is too low, the paint flows too easily.
Sagging, insufficient film thickness, poor hiding, and uneven color may occur.

Viscosity therefore needs to be controlled.

However, viscosity is only an indicator of how easily paint flows.
How much coating film will remain after drying?
Can the required film thickness be achieved?
Will the coating film develop sufficient performance?

These questions cannot be answered by viscosity alone.

Thinning involves more than making paint easier to apply

Thinning means adding thinner or water to paint to adjust it to a condition suitable for application.

On the shop floor, paint is thinned to improve application properties.
This is perfectly natural.

However, thinning is more than a simple adjustment of application properties.

A change in thinning ratio changes the proportion of the components in the paint that become the coating film.

For example, adding more thinner makes paint less viscous.
It may become easier to spray.
However, even if the same quantity is applied, fewer coating film components remain after drying.

As a result, the required dry film thickness may not be achieved.

In other words, thinning ratio is a control parameter directly linked to coating film quality as well as application properties.

Solids are the components that remain as the coating film after drying

Paint contains components that remain after drying and components that evaporate and disappear.

The components remaining after drying are called solids or nonvolatile content.
These are the coating film-forming components, including resins, pigments, and some additives.

Most solvents and water, on the other hand, evaporate after application.
They make the paint easier to apply and assist coating film formation, but generally do not remain in the final coating film.

This point is extremely important.

The amount of paint in the can is not the same as the amount of coating film remaining after drying.

Immediately after application, film-forming components and volatile components are present together.
After drying, the volatile components have escaped, and the solids remain as the coating film.

Understanding solids content is therefore necessary to stabilize film thickness.

Consider wet film thickness and dry film thickness separately

Film thickness can refer to wet film thickness or dry film thickness.

Wet film thickness is the thickness while the film is still wet immediately after application.
Dry film thickness is the thickness of the coating film remaining after drying.

What ultimately matters on the coating shop floor is dry film thickness.
This is because the coating film remaining after drying protects the product.

However, stabilizing dry film thickness requires an understanding of wet film thickness, solids content, and thinning ratio.

For example, even when paint is applied at the same wet film thickness, a different thinning ratio changes the thickness remaining after drying.
Paint containing more added thinner leaves less solids after drying, so its dry film thickness may be lower.

In other words, films that look the same during application are not necessarily the same after drying.

The same viscosity does not necessarily mean the same film thickness

On the shop floor, operators try to standardize application conditions by keeping viscosity constant.

This is important.

However, the same viscosity does not necessarily produce the same film thickness.

There are several reasons for this.

The first is a difference in thinning ratio.
If more thinner has been added to reach the same viscosity, the amount of solids remaining after drying changes.

The second is a difference in thinner type.
The thinner affects solvency, evaporation rate, leveling, surface texture, and drying characteristics.

The third is a difference in paint solids content.
Even at the same viscosity, high-solids and low-solids paints leave different film thicknesses after drying.

The fourth is a difference in application method.
Changes in spray gun nozzle size, air pressure, delivery rate, gun distance, or application speed change film thickness.

In other words, viscosity is important, but film thickness cannot be controlled by viscosity alone.

Thinner is more than a diluent

Thinner is used to dilute paint.

However, that is not its only role.

Thinner affects the following quality factors.

• Viscosity
• Solubility
• Drying rate
• Leveling
• Sagging
• Blushing
• Pinholes
• Surface texture
• Adhesion
• Coating film formation
• Working environment
• Safety

For example, a thinner that evaporates too quickly may cause the surface to dry rapidly while solvent remains inside the coating film.
This can lead to a rough finish, blushing, poor adhesion, or blistering.

Conversely, a thinner that evaporates too slowly may delay drying and cause sagging, dirt pickup, blocking, or drying defects.

Using a thinner whose solvency is unsuitable for the paint may also prevent the resin from dissolving sufficiently, potentially causing separation, settling, appearance defects, and coating film defects.

A thinner should not be chosen simply because it is inexpensive and readily available.
It must suit the paint and the process.

What happens when the thinning ratio is increased too much?

Increasing the thinning ratio may improve application properties.
However, excessive thinning carries risks.

Examples include the following problems.

• Insufficient dry film thickness
• Insufficient hiding power
• Uneven color
• Insufficient rust prevention
• Reduced durability
• More application passes
• Paint consumption increases
• Increased thinner consumption
• Changed drying conditions
• Greater tendency to sag
• Greater quality differences between operators

At first glance, adding more thinner may seem to save paint.
However, failure to achieve the required film thickness can lead to recoating or defects.

As a result, paint costs, thinner costs, working time, inspection labor, and rework costs may increase.

Excessive thinning can create a quality risk instead of reducing costs.

Insufficient film thickness affects function as well as appearance

Insufficient film thickness is not just a visual problem.

Insufficient film thickness may cause the following problems.

• The underlying surface shows through
• Color is inconsistent
• Gloss is inconsistent
• Insufficient rust prevention
• Insufficient water resistance
• Insufficient chemical resistance
• Insufficient weather resistance
• Premature deterioration

Achieving the required film thickness is particularly important for coatings intended to provide rust prevention or durability.

A coating film that is too thin cannot provide its intended protective function.

Film thickness control is therefore a foundation of quality assurance, as well as appearance control.

Excessive film thickness also causes problems

Film thickness causes problems when it is too thin, but also when it is too thick.

Applying excessive thickness may cause the following defects.

• Sagging
• Drying defects
• Insufficient curing
• Blistering
• Cracking
• Poor adhesion
• Surface wrinkling
• Solvent retained inside the film
• Increased costs

In particular, excessive thickness in hard coating films or chemically curing paints can increase internal stress and lead to cracking or peeling.

For baking paints and powder coatings, excessive thickness may also make it difficult to achieve sufficient curing throughout the film.

Greater film thickness is not always better.
Each paint and application has an appropriate film thickness.

Average film thickness alone is not enough

Looking only at average film thickness is risky when controlling film thickness.

Even if the average is within specification, some areas may be too thin or too thick.

The following areas are particularly prone to film thickness variation.

• Corners
• Edges
• Around holes
• Recessed and raised areas
• Back surfaces
• Welded areas
• Around bolts
• Inside complex shapes

Rust may start in thin areas.
Drying defects or cracking may occur in thick areas.

Film thickness control therefore needs to check variation as well as the average.

Which areas tend to be thin?
Which areas tend to be thick?
Are measurement locations standardized?

It is important to check these points.

Application conditions also affect film thickness

Film thickness is not determined by paint and thinning ratio alone.
Application conditions also have a major influence.

Examples include the following conditions.

• Spray gun nozzle size
• Air pressure
• Delivery rate
• Gun distance
• Gun angle
• Application speed
• Overlap width
• Number of application passes
• Workpiece shape
• Hanging arrangement
• Operator movements
• Air velocity inside the booth
• Electrostatic conditions

Even with the same paint and thinning ratio, film thickness changes when application conditions change.

Stabilizing film thickness therefore requires standardized application operations as well as paint control.

Record thinning ratio, viscosity, and film thickness together

Records are important for stabilizing coating quality.

The following items in particular need to be controlled together.

• Paint batch
• Thinner used
• Thinning ratio
• Viscosity
• Measurement temperature
• Type of viscosity cup
• Spray conditions
• Wet film thickness
• Dry film thickness
• Drying conditions
• Operator
• Defect occurrence

When defects occur, tracing the causes becomes difficult without these records.

With records, however, differences in thinning ratio, changes in thinner, variations in film thickness, and changes in drying conditions can be checked.

It is important to base coating control on records rather than relying solely on intuition.

Organizing thinning ratio and film thickness control through the 4Ms

CoatureVietnam also organizes thinning ratio and film thickness problems through the 4Ms.

Material: materials

• Has the paint’s solids content been confirmed?
• Is the specified thinner being used?
• Is an alternative thinner being used?
• Has the paint batch changed?
• For a 2-component paint, is the mixing ratio correct?

Method: methods

• Has a standard thinning ratio been established?
• Is the viscosity measurement method consistent?
• Is the temperature during viscosity measurement controlled?
• Is the number of application passes standardized?
• Have film thickness measurement locations been defined?
• Do the drying conditions suit the film thickness?

Machine: equipment

• Is the spray gun condition consistent?
• Are there any abnormalities in the pump, hoses, or nozzle?
• Is booth air velocity appropriate?
• Is the drying oven temperature distribution stable?
• Is the film thickness gauge calibrated?

Man: people

• Do operators make different judgments about thinning?
• Are there variations in the viscosity measurement method?
• Is training provided in film thickness measurement?
• Are there rules for reporting abnormalities?
• Is recordkeeping an established practice?

Organizing these issues through the 4Ms allows thinning ratio and film thickness problems to be treated as process-control issues rather than matters of individual operator intuition.

Points to check on the shop floor

To stabilize thinning ratio and film thickness, it is important to check the following points.

• Has a standard thinning ratio been established?
• Is the actual thinning ratio recorded?
• At what temperature in °C is viscosity measured?
• Is the same type of viscosity cup used consistently?
• Is the same thinner used every time?
• Are there approval rules for using an alternative thinner?
• Is the dry film thickness specification clear?
• Is wet film thickness also controlled?
• Have film thickness measurement locations been defined?
• Is variation checked as well as average film thickness?
• Have areas prone to insufficient or excessive film thickness been identified?
• Is the film thickness gauge calibrated?
• When defects occur, are records of thinning ratio, viscosity, film thickness, and drying conditions available?

How CoatureVietnam can help

CoatureVietnam supports stable coating quality not only by supplying paints and thinners, but also by considering thinning ratio, solids content, film thickness, and drying conditions.

For example, we can provide the following support.

Thinner Streamer
We support the selection of thinners suitable for the paint, the review of alternative thinners, and the clarification of drying and application properties.

Paint Optimizer
We review the thinning ratio, film thickness, drying conditions, and application methods of existing paints to support stable quality and cost improvements.

Operating standards and process control
We support the development of SOPs for thinning ratio, viscosity measurement, film thickness measurement, and drying conditions to reduce variation between operators.

Coat Visioner
We organize film thickness data, defect trends, and thinning ratio records to make quality visible.

Summary

Viscosity control is important.
However, viscosity alone cannot adequately control coating quality.

To stabilize coating quality, the following items must be considered together.

• Thinning ratio
• Solids content
• Thinner used
• Viscosity
• Wet film thickness
• Dry film thickness
• Film thickness variation
• Application conditions
• Drying and curing conditions

Adding more thinner to make paint easier to apply will not stabilize quality unless the required coating film is achieved.
Even when viscosity is correct, changes in solids content or film thickness change coating film performance.

What matters is checking not only ease of application, but also whether the required coating film has been achieved after drying.

Coating the Future with Science and Care
CoatureVietnam supports consistent coating film formation by considering thinner, thinning ratio, solids content, and film thickness together.