Spray Painting

Once the compressor system has been set up paint spraying can then be undertaken.  To achieve a smooth even coat the gun and the material must be prepared and thinned in accordance to the manufacturer’s guidelines.  The gun must be clean and properly loaded, the airflow and fan pattern adjusted and the compressor set at the correct pressure.

It is recommended to make a note of the thinning ratio & to run one or more test on a similar surface before starting with the actual project.

Many materials that can be applied with a brush need to be thinned down for paint spraying.  The thickness of the material is known as the “viscosity” and by adding thinning material such as water or thinners lowers the viscosity enabling the material to be atomised.

First the type of thinning agent must be determined which most manufacturers will include on the material container.  In some cases the manufacture will state that no thinning agent is required or will provide the ratio of thinning agent to material.  An excellent guideline is to use the same material that is recommended to clean up.  For example if the can says clean the brush in water then use water as a thinning agent.  If the can suggest the brush is to be cleaned with mineral spirits than use this material as a thinning agent.  Using measuring cup and a separate mixing vessel is desirable, as this will allow subsequent mixtures to be almost identical to the initial mixture.  Mixing the material in a separate vessel ensures the correct viscosity before being poured into the spray gun pot.

Using too much thinner will cause a number of problems.  The material being applied to the surface may run, drip or sag.  Even if this does not occur more coats may need to be applied to achieve the desired coverage and finish.  Also because the coats will dry much quicker pinprick bubbles may form as the excessive amount of thinners try to escape.  On the other hand not enough thinners will result in either a blocked spray gun or a very coarse or lumpy surface. 

When pouring the mixture into the spray pot it is vital that the pot is clean, dry and free from any dust as all these impurities will be transferred onto the spray surface.  Paint strainers can also be used to remove any impurities that may have gathered during the mixing process.  There are many to choose from, however the most popular and widely available are nylon mesh or paper funnels.

Stirring the paint

Using a paint strainer

Now that the material is strained and loaded and the gun is properly adjusted; you are now ready to spray.  Using a spray gun is not very difficult providing a few techniques are followed.  Plenty of practice is required before pointing the material at an expensive or delicate piece of furniture.

To achieve a smooth even coverage follow these simply guidelines which appertain to any piece of work:

Where possible place the work on rollers or a turntable so the same spraying position is maintained throughout.  If this is not practical place the piece of work in a position where minimum repositioning of the work is needed.  For example when spraying a pine chair, place this in an elevated position so that all the surface areas are accessible without moving.  Hold the gun perpendicular to the work surface at all times and at the same distance from the work for the entire “pass”.  A “pass” describes the action of spraying from one end of the work to the other where a series of passes is needed to complete the entire piece.  Again it is advisable to practice these techniques on waste wood, cardboard or paper first.

Although the order in which the parts of a piece is work will depend on the size and shape it is important to always try and spray with the gun moving away from the body and towards the exhaust system such as open garage door, repellent fan, exhaust fan or when spraying outdoors into the direction of the wind.  Before spraying it is good practice to make a dry run of the entire process.  Start with the least visible areas and work towards the parts that will be seen.  This will create an even finish and allow you to work around spots that may already have been covered.  For instance when spray tabletops or kitchen cupboards it is important that the edges are sprayed first.  In tight areas such as draws more wrist action is required and in some cases “feathering” or spraying lightly is required.  When you become more familiar with the equipment you will develop a feel for how the gun responds and become more adept at controlling where and how heavily the finish is applied.

THE MECHANICS

When spraying move your arm in a wide sweeping motion whist keeping your hand as still as possible and carry the gun in a straight line (“pass”) across the work.  The best way to obtain a smooth even pattern is to begin spraying about 6 inches before the work and 6 inches after the work and then release the trigger.  After a few passes this will become second nature and the small amount of material lost will be insignificant.

Before you begin to spray adjust the gun to the widest fan as possible to reduce the amount of passes needed to complete the work.  The gun should be held approximately 6 to 8 inches away from the work.  When spraying smaller pieces reduce the size of the fan and spray slightly closer to the work.  You will quickly ascertain the correct size of fan and the distance to hold the gun for each piece of work.  It is important to move the gun at the same speed as a brush stroke, however if the material is going on too light or too heavy then you must increase or decrease the speed of the pass.  A pass should overlap the previous pass by approximately 50% or 100% if the previous pass is a light tack coat.  A pass should also follow the same direction as the previous one to avoid criss-crossing as this can result in uneven coverage.

Correct                          Incorrect

CLEANING THE SPRAY GUN

It is essential that the entire spray gun is cleaned thoroughly after each project and when changing the type of material being sprayed.  Always use the same material that the paint was thinned with to clean the gun, e.g. water, mineral spirits, etc.  The most common problem is a blocked spray gun caused by dried paint being present in the tip, nozzle or siphon tube.  Remember, when painting conventionally a brush that is not cleaned correctly results in unnecessary work when used again or in some cases having to be discarded.

After completing the project the compressed air supply should be turned off at both the regulator and pressure switch.  Point the gun into an empty container and release any remaining compressed air that is present in the hose.  Remove the cup/pot carefully and empty any remaining liquid into a suitable container; the gun can be put to one side for the moment.  The pot should then be cleaned thoroughly with the appropriate solvents.  Remember any paint that remains will dry quickly and be much more difficult to remove later.

Once the pot is completely paint free pour in a small amount of solvent and return it back to the gun.  Any lids or gaskets should also be cleaned before sealing the pot back onto the spray gun.  The nozzle assembly can also be cleaned at this stage.

When the gun has been re-assembled and attached to the hose, turn on the compressor and release the compressed air.  Direct the gun towards an empty container and spray the solvent through the gun.  For a few seconds the solvent will emerge with the material that was previously sprayed, however, this will quickly disappear and pure solvent will be present.  The spray gun is now completely paint free and can be stored away.  Please remember to remove the solvent from the pot using the same method as removing the paint.

 
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