If your paint is cracking, bubbling, blistering or sagging, what could be the cause? In our guide to paint failures, we look at the reasons why a paint job may not go to plan.
The recipe for problem-free painting
Paints and coatings are comprised of an often complex combination of raw materials, and they require correct application in order to perform well. To get the best results, users must prepare and apply the paint properly, and then let it dry and cure in accordance with the specifications.
Just as importantly, users must prepare the substrate (the material to be painted) carefully for the paint to perform to its maximum capability. When done correctly, the paint will adhere well and provide the specified protection against the elements. But if mistakes are made, paint failures can happen, and the integrity and aesthetic qualities of the paint will suffer.
Paint failures can take several forms and can happen at various times in the life of a coating. Runs and sags may be seen during application, orange peel shortly after, and blistering and rust spotting later still.
Why has your paint failed?
It can be difficult to establish the reason for a paint failure, partly because one or more of several factors may have contributed to it. Therefore, in order to establish the cause of a paint failure, all possible contributory factors must be evaluated. That can mean establishing a detailed history of the surface preparation, mixing, application, curing and any other relevant factors up to when the failure was first noted.
Detailed below is a brief list of the most common causes of paint and coating failures, followed by a more detailed table showing types of failure and possible causes. This information should help you work out what the defect is, possible causes, how to rectify the problem, and how to avoid it happening again.
The paint itself
Modern manufacturing methods and quality control systems mean that it is unlikely that the paint when supplied is faulty. More likely factors that affect the quality and performance of the paint include:
- The paint has been kept way beyond its shelf life (typically 12 months).
- The paint has been stored at temperatures below or above the manufacturer’s recommendation. A good rule of thumb is to store paint at or near room temperature (around 20 degrees centigrade).
- The paint is a two-pack paint product that was not mixed at the correct ratio.
- Too much or too little thinner was added.
- The paint was not stirred sufficiently.
With regards to stirring, unless the manufacturer specifically warns against stirring, it is generally advisable to stir thoroughly. In the case of two pack paints, stir each component thoroughly before mixing, and then stir the mixture to blend them fully.
Giving the can a quick shake is rarely enough. Pigment and other solids can settle and need to be fully reincorporated into the paint through stirring. If this is not done, the paint’s expected properties may be compromised.
Paint applied to a contaminated surface
Paint adhesion can be badly affected by contamination on the surface. Good surface preparation will remove this contamination, but if surface preparation is hurried or not done correctly, contamination may cause problems. Forms of contamination include:
- Dust and dirt.
- Traffic film.
Rustbuster’s system for surface preparation includes cleaning and degreasing metal surfaces first: our own degreasers are SP-10 Tank Kleen and Safer Degreaser. Detergents and degreasers should be properly neutralised with water before painting.
We would also recommend using a salt remover as part of the cleaning process, especially for vehicle underbodies, and metal structures located near the sea. Our product is Chlor-X Salt Remover.
As a final step before applying paint, a specialist panel wipe is usually the right choice. White spirit and thinners are generally not suitable as they are likely to leave a greasy film on the surface. Rustbuster offers a Pre-Paint Panel Wipe that removes the last traces of grease to improve adhesion of the paint.
Poor application technique
Applying paint too thickly or thinly can of course create problems. If in doubt, err on the side of applying multiple coats thinly rather than trying to achieve the desired final thickness in one coat.
- Other potential application mistakes include:
- Applying a second coat outside of the recommended overcoating interval.
- Incorrect set up of spray equipment.
- Spraying to close or too far from the surface.
- Applying paint onto a gloss surface without keying first.
Painting in unsuitable environmental conditions
The main issues here are temperature and humidity:
- Temperature. If paint is applied below the paint manufacturer’s specified minimum, it may not adhere or cure correctly.
- Humidity. Avoid painting onto steel that is subject to condensation. Achieve this by ensuring that the steel is at least 3 degrees centigrade above the dew point. Read our guide to dew point calculation here.
Note also that changes in temperature or humidity during the drying/curing time can also create problems. A common example is when someone paints at a suitable temperature, but then it gets significantly colder during the first phase of drying and curing. Temperatures at or above the manufacturer’s recommended minimum should be maintained for at least the first few hours after application.
Your detailed guide to the causes of paint failure
What follows is a comprehensive guide to more than 20 types of paint failures, with the likely causes listed along with remedies and ways to prevent the failure happening again.
|Algae or mould (green or black growth on the surface).||Damp environment.||Usually treated with biocidal agents, scrubbed off or high-pressure water washed.|
|Bittiness (rough surface).||Wrong paint storage conditions. Coatings used out of date.||Contact coating supplier to confirm.|
|Bleeding (spotted discolouration).||Solvent attack from an underlying coating.||Apply an intermediate tie-coat/blocking primer to affected area.||Ensure compatibility of tie-coat/blocking primer with subsequent coatings.|
|Blistering (blisters or bubbles in the paint).||Liquid vapour gas under or within the paint film.||Remove affected area down to a sound substrate and re-coat.||Ensure complete removal of cause of blister, e.g. salts, acid.|
|Blooming (dull patches on the surface).||Condensation forming on partially cured coating – often when temperatures drop significantly during the first phase of curing.||Lightly abrade and apply another coat, allowing it to cure in warmer and drier conditions.||Blooming generally affects just the aesthetics of a coating and its performance is otherwise unaffected.|
|Chalking (white, chalky powder on the surface).||Exposure to harsh environment or UV light. Age of coating.||Remove affected area down to a sound substrate and re-coat.|
|Cissing (freshly applied paint recedes from the surface leaving small craters or bare areas).||The usual cause is contamination of the surface by grease, oil, wax, silicone etc. Cissing can also occur when overcoating glossy surfaces with no preparation.||Remove affected area down to a sound substrate and re-coat.|
|Cracking (cracked/flaky paint).||Curing rates, age of coating, what the coating was applied over.||Remove affected area down to a sound substrate and re-coat|
|Cratering (small, round recesses/depressions).||Air entrapment, gas release, solvent release, bubbles bursting.||Remove affected area down to a sound substrate and re-coat|
|Runs, tears, sags, curtains (sag over a significant length).||Applicator’s coating technique resulting in over application and coating applied too thickly.||If coating still wet, spread out the coating. If dry, remove/abrade and reapply.||A wet film thickness (WFT) gauge will help apply future coats to the correct thickness, using the coating’s technical data sheet as a guide.|
|Dry spray (dull appearance and rough surface).||Incorrect spraying technique. Ambient weather conditions unsuitable, e.g. too hot.||Remove affected area down to a sound substrate and re-coat.||Failure to remove a ‘dry spray’ primer will result in poor intercoat adhesion.|
|Flaking (lifting of the coating in the form of flakes or scales).||Adhesion failure due to contamination and poor surface preparation, or the overcoating time was exceeded.||Remove affected area down to a sound substrate and re-coat.|
|Flocculation (signs of small lumps in surface).||Possibly an issue with the paint itself.||Contact coating supplier with batch number to validate.|
|Grinning (visibility of the surface due to insufficient opacity of the paint).||Insufficient coating applied.||Apply further coating.||Establish the manufacturer’s recommended overcoating time and abrasion specification before overcoating.|
|Grit inclusions (fine particles in the paint).||Surface preparation undertaken too closely to painted areas. Surface not clean prior to application.||Remove affected area down to a sound substrate and re-coat.||Avoid recurrence by painting in a cleaner, enclosed area.|
|Holidays (gaps in the coating surface).||Poor application technique. Difficult access to areas being coated.||Prepare uncoated areas and apply further material.|
|Lifting (softening, swelling or separation of a coat after a second coat is applied).||Material drying too quickly. Solvent incompatible with existing paint. Overcoated too quickly.||Remove affected area down to a sound substrate and re-coat.|
|Orange peel (textured imperfection).||Poor application technique. Incorrect coating viscosity.||Correct the technique and/or adjust equipment.||If using a roller, try a shorter pile to not hold so much paint.
|Pinholing/rust spotting (small holes in the paint surface, often with rust spots showing through).||Contamination. Gas/solvent release possibly due to further coats being applied too soon.||Remove affected area down to a sound substrate and re-coat.|
|Residual tack (coating is still tacky).||Insufficient drying time. Low temperatures during drying time. Insufficient ventilation. If using a 2-pack incorrect mix ratio.||Allow full curing time as per the coating’s technical data sheet.|
|Ropiness (brush marks have not flowed out).||Material used outside of open time.||Remove affected area down to a sound substrate and re-coat.|
|Saponification (patches peeling from the surface).||Alkaline deposits remaining on the substrate||Remove affected area down to the bare metal substrate, remove contamination, re-prepare and re-coat.|
|Wrinkling (the paint film shrinks, causing a wave-like appearance).||Incompatibility of solvents with previous coats. Over-thick areas skinning. Wrong intercoating interval. Drying rates too fast.||Remove affected area down to a sound substrate and re-coat|
|Zinc salts (typically a white haze on the coating).||Exposure to environment.||Remove by hand abrasion followed by washing down, re-preparation and re-coat.|