Common problems with paint application

rusty metalHelping you achieve the best results

Statistics show that as much as 95% of all coating failures are a result of poor surface preparation and application. Coating failures may appear during application, at the stage of curing or after a certain period of service life.

Good routines and correct application are essential to achieve long-term protection. Chris is on hand to advise you which are the best products for the job you want to accomplish and help you get the best results for your vessel.

Bird & Goodwin can help you ensure you do not compromise on corrosion protection

Below you will find some examples of common coating failures and the reason why they occur. Please note that there may be many reasons for a coating failure and in some cases it requires a lot of experience to find the exact cause.

Sagging

Paint sagging occurs when:

  • Paint is applied in excess of the DFT specified
  • Too much thinner has been added to the paint
  • The gun is held too close to the surface

Sags are recognised as "curtains" on the painted surfaces. If the wet film thickness is too high, excessive sagging can result in pools of paints forming on horizontal surfaces or in corners. After curing, the paint may crack all the way to the substrate in such areas and reveal unprotected steel.

If sagging is noticed at the spraying stage, it should be brushed out while the paint film is still wet. Repairs after drying consists of abrasion (sanding) and re-coating.

Blistering

blistering paintThis is one of the most common types of failure related to the adhesion of the paint. Sometimes the blisters are dry and sometimes filled with liquid. The blisters can be both large and small, often shaped as hemispheres. The size usually depends on the degree of adhesion to the substrate, or between the coats, and the internal pressure of the gas or the liquid inside the blister.

Blistering can be caused by a number of different conditions:

  • Soluble salts contaminating the substrate or contaminating the surface between coats. No coatings are 100% water proof. The moisture vapour passing through the coating can dissolve salt into a concentrated solution. Pressure in the high concentration liquid will cause blisters. This phenomen is called osmosis
  • Contamination of the surface (e.g. oils, waxes, dust, etc.) will not allow proper adhesion of the coating. The moisture vapour tends to be concentrated in these areas of low adhesion. In this case, the blisters are so-called "dry" blisters
  • Poor or inadequate solvent release from the coating. Entrapped solvents can increase the water absorption and moisture vapour transmission of the coating and lead to blistering. Solvent odour is usually connected with retained solvents. If the blistering is widespread on a construction: reblast and wash before a new system is applied. For local areas: blast or carry out other mechanical cleaning before recoating.

Lifting

Lifting is a raising of the undercoat. It is caused by a stronger solvent in the topcoat attacking the previously applied film. The result is a wrinkled surface. A typical example is a topcoat containing xylene, on top of an alkyd-based primer containing white spirit. The xylene in the topcoat will dissolve the primer.

Blast cleaning and reapplication of the paint is necessary to repair the damaged surface.

Delamination / Peeling

peeling paintLoss of adhesion to the substrate or between coats of paint is delamination or peeling. The causes are:

  • Unsatisfactory surface preparation
  • Incompatible primer or undercoat
  • Substrate or intercoat contamination
  • Excessive cure time between coats

The way to repair the surface, is to remove paint down to sound paint or to the substrate, and recoat.

Orange peel

Finely pebbled or dimpled surface texture with an appearance similar to the skin of an orange. Caused by:

  • Improper atomisation due to low air pressure
  • Spraying too close to the surface
  • Rapid solvent evaporation

Orange peel is mainly a cosmetic defect; sand down to smooth surface and repaint if necessary.

Pinholes & Pores

Using the wrong spraying technique, such as excessive air pressure, excessive film thickness, strong wind (too good ventilation) and too long application distance may cause craters, pinholes and pores. If noticeable on the paint film, check the spraying equipment to ensure that the air pressure and nozzle size is correct.

Pinholes in a paint film can also result from overspray. On excessive film thickness air will be entrapped in the paint. The escaping air will create pinholes. The consequence being that pinpoint rusting occurs, followed by undercutting of the coating around the pinholes.

Repairs consist of grinding and re-coating the area using appropriate coats to seal the defects and build up the coating to the correct DFT. If the coating is still uncured, brush out and apply the additional coat.

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