Checking the underlying coating film is important in repair coating and recoating

After coating, the film shrinks into wrinkles.
The surface rises.
The coating film appears lifted.
Localized wrinkling occurs.
Only recoated areas develop a rough surface.
Abnormalities appear only around repaired areas.

Such defects are sometimes called lifting.

Rather than a defect in the newly applied topcoat paint itself, lifting is a defect caused by solvent in the topcoat attacking the primer or existing coating film.

Repair coating and recoating require particular attention.

Even with the same new paint, the finish changes substantially according to the underlying film type, drying condition, deterioration, and solvent resistance.

The fundamentals of preventing lifting therefore involve more than examining the topcoat. The key is checking what lies underneath.

1. What is lifting?

Lifting is a defect in which topcoat solvent attacks the underlying film after topcoating, causing it to lift, shrink, or wrinkle.

It may appear in the following forms.

• The surface appears shrunken
• The surface appears wrinkled
• The coating film is lifted
• Only some areas appear blistered
• The surface around repaired areas becomes rough
• Abnormalities appear at the boundary with the old film
• Surface texture is disrupted only in recoated areas

This defect may be visible immediately after application or emerge gradually during drying.

The cause is topcoat solvent entering the underlying film and softening it again or making it swell.

When the underlying film moves, the new film above moves with it and the surface becomes wrinkled.

2. Lifting is not solely a topcoat problem

When lifting occurs, topcoat paint tends to be suspected.

In practice, however, examining the topcoat alone cannot identify the cause.

The relationship between the topcoat and underlying film must be checked.

For example, strong solvents in the topcoat are more likely to attack the underlying film.
An insufficiently dried primer is also more easily disturbed by topcoat solvent.
An existing lacquer film or a film with weak solvent resistance is more readily attacked during recoating.
Even a 2-component paint may not have developed sufficient solvent resistance if it is undercured.

Lifting therefore needs to be viewed as a compatibility problem between topcoat solvent and the underlying coating film, rather than simply a topcoat problem.

3. Conditions that make lifting more likely

Lifting is more likely under the following conditions.

• The primer is insufficiently dried
• The existing coating film type is unknown
• The existing coating film has weak solvent resistance
• Paint containing strong solvents is applied over an existing lacquer film
• A 2-component paint is undercured
• The hardener mixing ratio is inaccurate
• Paint is used beyond its pot life
• The topcoat thinner has excessive solvency
• The topcoat is applied too thickly
• Insufficient sanding leaves the old film surface unstable
• Filler or surfacer is insufficiently dried
• The boundary between the repair and old film is inadequately prepared
• Application falls outside the recoating window

In repair coating particularly, the paint used for the old film may be unknown.
Applying a topcoat with strong solvents in this state can cause lifting.

In repair coating, the important question is whether the underlying film can withstand topcoat solvent, rather than simply whether paint can be applied.

4. Insufficient primer drying is a major cause

Insufficient primer drying is one of the typical causes of lifting.

If topcoat is applied before the primer has dried or cured sufficiently, topcoat solvent enters the primer.
If the primer is still soft, the solvent makes it swell again or move.

As a result, the surface shrinks together with the topcoat film and lifting occurs.

In such cases, changing topcoat paint may not resolve the underlying problem.

The primer drying conditions need to be checked.

• Is sufficient time allowed after priming?
• Is drying temperature appropriate?
• Is workpiece temperature sufficient?
• Is film thickness excessive?
• Is the primer thinner too slow-drying?
• Is the primer film dried throughout its interior?
• Is the recoating window observed?

The primer is the foundation of the topcoat.
Without a stable foundation, the topcoat will not be stable either.

5. Recoating without knowing the old film type is risky

The old film type is extremely important in recoating.

Which paint was used for the old film?
How old is it?
Has it deteriorated?
Does it resist solvent?
Is it firmly adhered?
Are there previously repaired areas?

Applying a topcoat without checking these points increases the risk of lifting.

The following old films require particular attention.

• Lacquer films
• Insufficiently dried old films
• Undercured 2-component films
• Films deteriorated with age
• Thick films with insufficient internal drying
• Films with an unknown repair history
• Areas with substantial filler or surfacer

If the old film type is unknown, full coating should not begin immediately.

Perform a small-area trial and check for lifting, wrinkling, bleeding, and poor adhesion.

6. Topcoat thinner solvency is also important

The solvency of the thinner in topcoat paint is important in lifting.

Excessively strong thinner solvency makes attack on the underlying film more likely.
If the old film or primer has weak solvent resistance, strong thinner softens its surface and causes the film to move.

A thick topcoat also increases the amount of solvent contacting the underlying layer.
This increases the risk of attacking the underlying film.

Measures against lifting require the following checks.

• Is the specified thinner being used?
• Is an excessively strong thinner being used?
• Is the topcoat excessively thick?
• Has compatibility with the primer been confirmed?
• Is solvency excessive for the old film?
• Does trial application show any problems?

Thinner is more than a material for diluting paint.
It is an important material affecting the underlying film.

7. Observe the recoating window

Coating specifications may define a recoating window.

This is the time range after priming during which the topcoat may be applied.

If application is too early, the primer remains soft and is readily disturbed by topcoat solvent.
If it is too late, the film surface may become excessively hard or adhesion may deteriorate.

Recoating therefore cannot be performed freely at any time.

The appropriate recoating interval changes with paint type, drying conditions, film thickness, temperature, and humidity.

The following shop-floor checks are important.

• Are the paint manufacturer’s recoating conditions checked?
• Is elapsed time after priming recorded?
• Is recoating being performed too early?
• Is topcoat being applied after a long delay?
• Is drying delayed by high humidity or low temperature?
• Is drying delayed only in thick areas?

The recoating window is an important condition for quality assurance.
Changing it solely for operating convenience may cause defects.

8. Pay particular attention to filler and surfacer areas

Repair coating may use filler or surfacer.

These areas can be particularly prone to lifting.

This is because filler and surfacer tend to be thick and their interiors dry slowly.
Solvent penetration may also differ at sanded surfaces and boundaries with old film.

If filled areas are topcoated while insufficiently dried, topcoat solvent or heat during drying can cause internal components to move and produce surface defects.

The following checks are needed in repaired areas.

• Has the filler dried for the specified time?
• Has it been applied too thickly?
• Is sanding dust removed?
• Is the surfacer sufficiently dried?
• Is the boundary with the old film properly prepared?
• Does trial application show any abnormalities?

If defects appear only in repaired areas, check filler, surfacer, and the old-film boundary before focusing on topcoat paint.

9. Actions when lifting occurs

If lifting occurs, trying to conceal it with further coats can cause recurrence.

This is because the unstable underlying film responsible remains in place.

For mild cases, the affected area may be sanded after thorough drying, then recoated once a stable underlying surface has been established.

For severe cases, the defective film must be stripped and the responsible layer removed before recoating.

Important treatment points are as follows.

• Do not immediately apply another topcoat
• Allow thorough drying before checking the condition
• Sand or strip the defective area
• Stabilize the underlying surface
• Use a sealer or surfacer as necessary
• Check for recurrence through trial application
• Record recoating conditions

Lifting may recur if only the surface is repaired.
It is important to stabilize the underlying layer causing the problem.

10. Particular points to consider in factories in Vietnam

Several conditions in factories in Vietnam can readily lead to lifting.

First is insufficient standardization of repair coating.
Proceeding with sanding, filler, surfacer, and topcoat based on ad hoc shop-floor judgment makes insufficient underlying drying and inadequate old-film checks more likely.

Next is insufficient information about the old film.
For subcontracted parts, recoated items, and previously repaired products, the paint underneath may be unknown.

Production speed may also take priority, shortening primer or filler drying time.
High humidity delays drying, so the normal period may be insufficient.

Thinner changes made by shop-floor judgment also require care.
Strong thinner may more readily attack the old film.

In factories in Vietnam, repair coating in particular needs SOPs.

11. Shop-floor checkpoints

When lifting occurs, check the following items.

Defect occurrence

• Which areas are affected?
• Is it confined to repaired areas?
• Does it occur at the boundary with the old film?
• Did it appear immediately after application or during drying?
• Is it across the whole surface or localized?
• Does it look wrinkled, lifted, or shrunken?

Underlying layer condition

• Is the primer sufficiently dried?
• Is the old film type known?
• Does the old film have solvent resistance?
• Has the old film deteriorated?
• Are filler and surfacer dry?
• Are there thick-film areas?
• Does sanding dust or contamination remain?

Topcoat conditions

• Is the specified thinner being used?
• Is thinner solvency excessive?
• Is the topcoat excessively thick?
• Is application viscosity appropriate?
• Is flash-off time appropriate?
• Are products entering the drying oven too early?

Recoating conditions

• Is the recoating window observed?
• Is elapsed time after priming recorded?
• Is drying delayed by high humidity or low temperature?
• Is trial application performed?
• Is suitability over the old film checked?

These checks make it easier to distinguish whether lifting comes from topcoat conditions or the underlying film.

12. Improvement perspectives CoatureVietnam can propose

CoatureVietnam does not view lifting solely as a topcoat defect.
We organize it through the 4Ms as a relationship between the underlying film and topcoat solvent.

Material: materials

• Are the topcoat solvents excessively strong?
• Is the specified thinner being used?
• Is the topcoat compatible with the primer?
• Is old-film solvent resistance sufficient?
• Are filler and surfacer drying characteristics appropriate?

Method: methods

• Is primer drying time observed?
• Is the recoating window controlled?
• Is the old film checked?
• Is trial application performed?
• Is excessive thickness avoided?
• Are defective areas stripped or sanded before recoating?

Machine: equipment

• Are drying oven conditions appropriate?
• Is workpiece temperature sufficient?
• Are sanding equipment and dust collection appropriate?
• Are temperature and humidity in the repair-coating area controlled?

Man: operators

• Do operators understand the importance of checking the old film?
• Can they distinguish lifting from an ordinary rough finish?
• Can they identify insufficient drying in repaired areas?
• Do they understand the need for trial application?

For Management, it is important to develop repair-coating SOPs, old-film check sheets, recoating interval controls, trial application criteria, and treatment criteria when lifting occurs.

Summary

Lifting is a defect in which topcoat solvent attacks the primer or old film, causing the coating film to lift, shrink, or wrinkle.

It may look like a topcoat defect.
However, its underlying issues are the lower film’s solvent resistance and drying condition, paint-system compatibility, and recoating conditions.

In repair coating and recoating, applying paint without knowing the old film type creates a major risk.

Measures include checking the old film.
Drying primer and filler sufficiently.
Using the specified thinner.
Avoiding excessive thickness.
Observing the recoating window.
Performing trial application as necessary.

Lifting is a defect that can be prevented through checks before application.

Coating the Future with Science and Care
CoatureVietnam does not leave repair coating and recoating solely to shop-floor judgment. We define old-film checks, recoating conditions, thinner compatibility, and treatment criteria to support coating processes less prone to recurrence.