Recoating is a task of diagnosing lower layers, not simply applying another coat

Repair coating or recoating is sometimes necessary on the coating shop floor.

The product has been scratched.
The color did not match.
Dirt attached to the surface.
Sagging occurred.
The coating film peeled off.
The customer requested rework.

In these situations, people on the shop floor tend to think that applying another coat will fix the problem.

However, repair coating has different conditions from coating a new substrate.

A coating film already exists.
More coating material is applied over this existing film.
The quality after recoating varies greatly depending on what type of coating it is, how thoroughly it has dried, whether it has deteriorated, and whether it can withstand solvents.

In other words, in repair coating,do not coat without knowing the type of existing film.

Recoating is not simply applying coating material on top.
It involves checking the film underneath and selecting an application method appropriate to its condition.

1. Repair coating is more difficult than initial coating

For initial coating, the substrate, surface preparation, coating material, and drying conditions can be designed from the beginning.

In repair coating, however, some form of coating film already exists.
If it is stable, there will be fewer problems.
If it is unstable, defects may occur regardless of how good the new coating material is.

For example, an insufficiently cured existing film may swell under the topcoat solvent and cause lifting.
If the existing film contains dye-based colorants, bleeding may occur after topcoating.
If it has deteriorated and become powdery, poor adhesion occurs.
If oil or wax remains on it, cratering or peeling occurs.

Repair coating is more difficult than it looks.

This is because information about the underlying film is often insufficient.

2. What happens when the existing film is unknown?

Coating without knowing the type or condition of the existing film causes various defects.

Typical defects are as follows.

• Lifting
• Wrinkling
• Bleeding
• Poor adhesion
• Cratering
• Dirt inclusions
• Gloss defects
• Drying defects
• Uneven color
• Sanding marks showing through
• Defects around filler edges
• Recurrence after another repair

Lifting is particularly common.

Solvent in the topcoat attacks the existing film, causing it to swell or soften.
As a result, the surface shrinks and wrinkles, and the film appears to lift.

If the existing film contains dye-based colorants or tar constituents, color may also rise from the lower layers after topcoating.
This is bleeding.

Repair coating defects therefore arise from the relationship with the existing film as well as the topcoat material.

3. What to check in the existing film

The existing film needs to be checked before repair coating.

First, identify its coating type.

• Is it a lacquer?
• Is it an alkyd coating?
• Is it an acrylic coating?
• Is it a urethane coating?
• Is it an epoxy coating?
• Is it a melamine baking coating?
• Is it powder coating?
• Is the coating type unknown?

Next, examine the film’s condition.

• Has it dried and cured sufficiently?
• Has it deteriorated?
• Has it become powdery?
• Is there lifting or peeling?
• Is adhesion sound?
• Is oil or wax present?
• Is sanding dust left on the surface?
• Does it contain moisture?

The repair history is also important.

• Has it been repaired repeatedly in the past?
• Has filler been applied thickly?
• Has the surfacer become thick?
• Are there many overlapping coating layers?
• Have some areas been repaired with a different coating material?

Coating without checking the existing film leads to recurring defects with unidentified causes.

4. Check the solvent resistance of the existing film

The existing film’s solvent resistance is particularly important in recoating.

Solvent resistance means whether the existing film can withstand the solvents in the new topcoat.

Topcoat materials contain solvents.
If these solvents enter the existing film, it may soften, swell, or dissolve.

If the existing film has poor solvent resistance, the following defects occur.

• Lifting
• Wrinkling
• Surface wrinkles
• Lifting of the existing film
• Reduced adhesion
• Gloss defects
• Uneven color

Particular care is needed with existing lacquer films, insufficiently dried films, and undercured two-component films.

If solvent resistance is unknown, it is important to perform a trial application on a small area rather than proceed directly to full application.

5. Trial application is an important check in repair coating

A trial application is particularly effective when the existing film is unknown.

It means applying the topcoat to a small area before full application to check for abnormalities.

Check the following points.

• Does lifting occur?
• Does wrinkling occur?
• Does bleeding occur?
• Does cratering occur?
• Is gloss stable?
• Are there adhesion problems?
• Do abnormalities appear after drying?

A trial application may seem like extra work.
However, it places a far smaller burden on production than defects across the entire surface after full application.

In repair coating, proceeding without understanding the existing film presents the greatest risk.
Check an unknown existing film through a trial application.
This is the basic rule.

6. Do not omit surface preparation

Repair coating does not mean that new material can simply be applied directly over the existing film.

Oil, dust, wax, sanding dust, skin oils, powder from deterioration, and dirt may be present on its surface.
Applying a coating without removing them causes cratering, poor adhesion, dirt inclusions, and peeling.

The following surface preparation steps are important before repair.

• Cleaning
• Degreasing
• Sanding
• Sanding dust removal
• Drying
• Removal of deteriorated coating
• Removal of lifted coating
• Sealer treatment where necessary

Residual sanding dust, in particular, causes poor adhesion and dirt inclusions.
Areas touched by hand may also retain skin oils.

In repair coating, quality is determined by surface preparation before application as well as application skill.

7. Pay attention to insufficient filler and surfacer drying

Filler and surfacer may be used in repair coating.

This stage also requires care.

When filler is applied thickly, its surface may be dry while the interior remains undried.
Thickly applied surfacer may also retain solvent internally.

If topcoating is performed in this condition, internal solvent affects the topcoat and causes the following defects.

• Pinholes
• Bubbling
• Bleeding
• Gloss defects
• Sanding marks showing through
• Lifting
• Drying defects

If defects occur only in repaired areas, the drying condition of filler and surfacer should be checked before focusing on the topcoat.

Check the following points.

• Has too much filler been applied?
• Has the specified drying time been observed?
• Is drying delayed by low temperature or high humidity?
• Is surfacer film thickness appropriate?
• Has sanding dust been removed?
• Has sufficient drying time been allowed before topcoating?

In repair coating, filler and surfacer are also part of coating film quality.

8. A sealer or intermediate layer may be necessary

If the existing film or underlying surface is unstable, applying a topcoat directly may cause defects.

In such cases, a sealer or intermediate layer may be used to stabilize the underlying surface.

A sealer is a coating film used to limit the influence of the underlying surface or existing film.
It makes it harder for topcoat solvent to enter lower layers.
It suppresses color migration from underneath.
It makes absorption uniform.
It stabilizes adhesion.

These are the purposes for which it is used.

However, a sealer does not solve every problem.
If the existing film is lifting or severely deteriorated, stripping or removal by sanding is required rather than a sealer.

The important decision is whether the existing film can be retained or must be removed.

9. Decide whether to retain or strip the existing film

Repair coating may be performed over the existing film or after stripping it and starting again.

If the existing film is sound, adheres well, and has solvent resistance, recoating may be possible after sanding and cleaning.

However, retaining the existing film itself becomes a risk in the following cases.

• The existing film is lifting.
• There is peeling.
• The film has deteriorated and become powdery.
• Adhesion is weak.
• The coating type is unknown and lifting occurs.
• It contains colorants that cause bleeding.
• Filler or repair layers are thick and unstable.
• Many coating layers have been applied over one another.

Even if these conditions are concealed with another coat, the problem may recur later.

Repair coating must consider prevention of recurrence as well as making a quick short-term repair.

This requires criteria for deciding whether to retain or strip the existing film.

10. Points requiring particular attention in Vietnamese factories

Repair coating presents several risks in Vietnamese factories.

First, the repair process tends to be left to individual operators.
If judgments about sufficient sanding or drying differ between operators, repair quality varies.

Second, information about the existing film is often insufficient.
For outsourced products, recoated products, imported components, and previously repaired products, the underlying coating may be unknown.

The hot and humid environment can also destabilize filler and surfacer drying.
Even if the surface is dry, internal drying may be insufficient.

Production deadlines may also take priority, leading to topcoating or packing immediately after repair.
This may cause lifting, bleeding, a return of tackiness, or pressure marks later.

In Vietnamese factories, it is important to establish repair coating as standardized work rather than rely on experience alone.

11. Shop-floor checkpoints

Check the following items before repair coating or recoating.

Existing film checks

• Is the existing coating type known?
• Has the existing film cured sufficiently?
• Does it have solvent resistance?
• Is there deterioration or powdering?
• Is there lifting or peeling?
• Have many coating layers been applied over one another?
• Is the previous repair history known?

Underlying surface preparation

• Are cleaning and degreasing sufficient?
• Is sanding appropriate?
• Has sanding dust been removed?
• Is oil or wax left on the surface?
• Is moisture left on the surface?
• Has it been touched with bare hands?

Filler and surfacer

• Has too much filler been applied?
• Has the filler dried all the way through?
• Is surfacer film thickness appropriate?
• Has sanding dust been removed?
• Is drying time before topcoating sufficient?

Topcoating conditions

• Is the topcoat compatible with the existing film?
• Is the specified thinner being used?
• Is the thinner’s solvency too strong?
• Is the coating applied too thickly?
• Is a trial application performed?

Treatment decisions

• Can the existing film be retained?
• Is stripping necessary?
• Is a sealer necessary?
• Will a minor repair be sufficient?
• Is full recoating necessary?

Checking these items helps reduce recurring defects in repair coating.

12. Improvement perspectives CoatureVietnam can propose

CoatureVietnam does not treat repair coating as simple rework.
It organizes it from a 4M perspective as a process that includes diagnosis of the existing film.

Material: Materials

• What type of coating is the existing film?
• Is it compatible with the topcoat?
• Is thinner solvency appropriate?
• Are the filler and surfacer appropriate?
• Is a sealer necessary?

Method: Methods

• Is there a checking procedure before repair?
• Is a trial application performed?
• Are sanding, degreasing, and cleaning standardized?
• Is the filler drying time observed?
• Are there criteria for deciding whether to retain or strip the existing film?

Machine: Equipment

• Is the sanding equipment appropriate?
• Is dust collection sufficient?
• Are repair coating area temperature and humidity controlled?
• Is the drying equipment suitable for repair coating?
• Is lighting sufficient to detect defects?

Man: Operators

• Do operators understand the importance of checking the existing film?
• Can they recognize signs of lifting or bleeding?
• Can they identify insufficient drying confined to repaired areas?
• Is there a culture of consistently performing trial applications?

From a Management perspective, it is important to establish repair coating SOPs, existing film check sheets, trial application criteria, filler drying standards, criteria for deciding whether recoating is possible, and post-repair inspection standards.

Conclusion

Do not apply repair coatings without knowing the type of existing film.

Recoating is not simply applying another coat.
It involves checking what coating lies underneath, its condition, and whether it can withstand the topcoat solvents.

Coating without checking the existing film may cause lifting, wrinkling, bleeding, poor adhesion, or drying defects.

In repair coating, it is important to check the existing coating type, deterioration, solvent resistance, filler drying, surfacer, surface preparation, and trial application results.

Repair so that the problem will not recur, rather than focusing only on speed.
This is the fundamental principle of repair coating.

Coating the Future with Science and Care
CoatureVietnam supports consistent repair quality by organizing existing film diagnosis, surface preparation, trial applications, and recoating criteria rather than relying solely on shop-floor experience.