Paint tins and cans kept on a construction site will eventually become dirty, particularly on top of the lid. It is important to remove such dirt before opening the lid, and to lift the lid away from the opening of the tin. If any dirt or contaminants fall into the paint, this may cause blockage of spray nozzles and/or degrade the dry paint film.

Low viscosity (thin) paints may suffer from some pigment settling at the bottom of the tin and need to be stirred before application starts.

Two-pack paints will always need to be mixed thoroughly before application starts. If the two components are not sufficiently blended, there will be areas in the mixture which will have the wrong proportion of base and curing agent. This will lead to incorrect drying / curing and weaknesses in the dry protective film.

Stirring paint by hand, using a wooden stick, will not give a satisfactory result, unless we are stirring a small tin (1 litre or less). Larger tins or cans must be stirred mechanically, using a power-mixer driven by compressed air or by an electric motor (electric motor is NOT recommended for solvent-containing paints). Ensure that whatever tool you use for stirring is clean, so it does not introduce impurities to the paint.

For two-pack paints, each pack should be stirred separately before the smallest pack is slowly added to the larger pack during additional stirring. Make sure all the content in the small pack is added, otherwise the mixing ratio will not be correct.

Paints are ready to be applied directly from the tin, the manufacturer has adjusted the viscosity to be at the optimum level, so no additional thinning is necessary. If thinner is added, this may lead to sagging during application, reduced hiding power, reduced dry film thickness, solvent entrapment in the film and longer drying time. In general, there are only three situations where addition of thinner should take place:

• The paint (and ambient conditions) is very cold, so the paint becomes too thick
• The ambient conditions (and substrate) is very warm, additional thinner will reduce dry-spray and keep the film open longer to ensure a continuous film
• If paint is applied on a very porous substrate, additional thinner for tie coat or mist coat application technique will aid penetration into the porous substrate

Never add an unspecified or unknown thinner to a paint. In addition to the above listed possible negative consequences, you will introduce an even worse possibility: the unspecified thinner may not be compatible with the binder in the paint. The result could be that the paint start gelling (which is easy to see), or some physical properties are changed (e.g. much longer drying time), or the resulting dry film is weakened (softer, less chemical resistance, etc.).

Corrosion can take place on any part of a steel structure, particularly in places where the corrosion protection is weak or damaged. During spay application of corrosion protective coatings, certain areas and positions may be difficult to reach or access with a spray gun, or the coating may tend to pull away from edges or refrain from entering tiny gaps, such as:

• Profiles
• Inside edges
• Holes, notches
• Corners, angles
• Sharp edges
• Pits (pitting)
• Manual welding seams
• Behind flanges

Stripe coated area

By applying one or two coats by brush before (or after) the spray application takes place, we will increase the thickness of the protective paint film in those vulnerable areas and thereby reduce the danger for corrosion. This is called “stripe coat”, since it predominantly ends up being paint applied in stripes on all the edges, welding seams, angles, etc.

A paint brush is an excellent tool for stripe coating, the brush wets the substrate properly and  works the paint well into the substrate. Use a good quality paint brush. Stripe coating will improve the life time of the paint system. Contrasting colours between the stripe coat and the full coat will make it easier for the painter to see where it has been applied and easier to inspect as well.

Importance of stripe coating welds

Paint application by brush can give quite good results provided a good quality brush is used and the correct technique is employed. The main advantage by brush application is the ability of the bristles to work the paint into any irregularities (profile) in the substrate. The disadvantages are that it is time consuming and the thickness of the film you can apply is limited.

Application technique - brush

• Wash a new brush with thinner
• Dry in a cool area
• Don’t dip the whole length of the bristle into the paint
• Move brush over the surface evenly
• Cross application method can form a uniform film thickness
• Push the brush to force paint to penetrate into voids or gaps
• Finish off by brushing in one direction to produce a more even film, colour and finish

Poor brush application can cause early breakdown of the paint. Brush marks (stripes) are a common problem and will lead to poor appearance and lower film thickness in the stripes.

Paint application by roller can be very useful and there are many different sizes available as well as the type and length of the “fur” (pile or nap) on the roller. In some situations only a roller will be able to deposit paint on the substrate, such as behind the flanges of a beam.

Application technique - roller

• Wash new roller with water or thinner prior to application
• Dry the roller in cool area
• Use a paint roller tray to load the roller fabric with paint
• Roll out the cover in the tray to remove excessive paint from the cover
• Cross application will result in a more even film thickness
• Finish off by rolling in one direction to produce an even film, colour and finish

Roller application will not ensure good wetting of the substrate or work the paint into the surface profile. Use of a roller for application onto bare steel is therefore not recommended, unless this is the only means of reaching the bare steel.

Roller Application

Airless spray application is the preferred method for most construction and maintenance industry. It is quick and cost efficient, can build up a high film thickness in individual coats and usually gives an attractive finish. The force of the airless spray will ensure a good wetting and penetration into the surface profile.

Application technique – airless spray

1. Setting up an airless spray pump:
2. Release the air pressure and trigger the gun to release any hose pressure then put the suction hose into the paint or fill a gravity hopper with paint
3. Force any residual thinner / cleaner out of the system at a pressure in the range of 1-2 kg/cm² until the loaded paint comes out of the hose
4. Mount the correct type of spray tip / nozzle to the airless gun, check all gun and hose connections are tight
5. Adjust the inbound air pressure until a correct spray pattern is achieved

Airless spray pattern

Rules when spraying

• Correct distance and angle from the gun to substrate to avoid dry spray (overspray). Dry-spray will develop at the edges of a wide spray fan. This may give a rough film and pinholes

• Seek to keep the gun at a right angle to the substrate. The distance should be between 30 and 60 cm. The optimal distance will vary, among other things with  wind, temperature, pressure at the nozzle and viscosity of the paint

• Keep correct overlapping between strokes to ensure an even film thickness
• The film thickness is affected by the distance between the gun tip and the surface, the speed of the gun stroke, and the pump pressure and nozzle size

Distance between gun and structure

• Keep the spray gun at a correct distance from the object
• A correct distance ensures a good, uniform paint film
• Too great a distance gives poor float and an uneven surface
• Trying to reach areas far away will give an uneven film thickness
• Shadows are created behind welds
• Waving the gun results in an unacceptable spray pattern

Longer distance between gun and substrate gives an increased surface roughness

• Increased risk of leaving dry spray on the surface
• Increased roughness of the inside of pipe walls will reduce the flow velocity
• A rough surface of the ship’s bottom will result in an increase in fuel consumption

Rules for application

• Start moving the spray gun before pulling the trigger
• Release the trigger before stopping the gun stroke
• Use smooth and even gun strokes
• Use as low air pressure as possible to obtain a uniform spray fan

When spray applying positions containing a hollow, such as an inside corner or into deep pits, a cushion of compressed air may form inside the hollow. This may prevent a sufficient amount of paint to be deposited at the bottom of the hollow area (inside corner / pit). Proper stripe coating in such areas is the best solution for this potential problem.

Airless spraying

Incorrect use of airless spray equipment will result in:

• Too much paint dust
• A rough surface
• Pinholes in the paint film
• Entrapped air
• Entrapped solvents
• High paint consumption

Time Aspects affecting paint

• Shelf Life: Acceptable storage time “on the shelf” in the paint store

• Pot Life: How long you can continue to apply a two-pack paint after the two components have been mixed

• Induction Time: Pre-reaction of the two components in the tin prior to application

• Drying Times: Time required from application until the paint becomes Surface dry/ Hard dry / Fully Cured

• Re-coating Interval: Minimum / maximum time after application before applying a subsequent coat