Using airless paint spray guns.
Handheld airless paint spray guns are available these days in AC mains electric powered and cordless battery-powered airless spray guns.
Check the owners handbook for the recommended nozzle size to suit the paint you plan to spray.
If your handheld airless spray gun has different filters as an option once again consult the owners handbook to select the correct filter for the paint and nozzle size you will be using.
Find a test area to test spray using the airless spray gun first and place a small amount of paint in the cup/container and spray a test piece.
If the atomisation from your handheld airless spray gun is not fine enough, for example there are heavier spots in the centre of the pattern and the spray pattern is not very wide, thin the paint approximately 5 to 10% at a time and test spray until acceptable atomisation and spray pattern is achieved.
For those using a viscosity cup this is the time to record the viscosity of the paint you have modified to achieve an acceptable paint spray pattern and atomisation.
Take the viscosity cup and immerse it completely in the modified paint and using a stopwatch time the seconds it takes for the viscosity cup to drain when it is lifted clear of the paint it was immersed in. Non thinned paint can now be accurately thinned to the right viscosity each time by rechecking new mixes with the viscosity cup to achieve the same time to empty.
Always thin and prepare smaller quantities of your paint at a time leaving non-thinned paint available to rectify if you happen to overshoot and thin a small batch too much.
Ensure the area to be sprayed with your handheld airless paint spray gun is clear, the surface prepared and any areas that need protecting our well masked.
It is usually best to begin spraying with an airless spray gun already moving across the surface and a little further away from the surface and then move the airless gun closer to the surface while maintaining the gun at 90° to the surface.
On broad areas it is usually better to slowly walk or reach up and squat down to achieve and maintain 90° to the surface. Standing still causes the airless spray gun to be moved in arcs which moves the airless spray gun at angles away from 90° to the surface which can cause uneven coverage and will contribute to overspray.
Many will find it easier to deliberately spray a much lighter coat for the first coat which will help them to see where they have sprayed and where they have not sprayed when applying the second coat.
Also with many oil-based enamel paints, polyurethanes and similar coatings applying a very light "tack" coat of 5 to 10 min before applying a subsequent full coat of paint will assist in avoiding runs and sags and aid in applying a heavier coat of paint that would normally be possible. Always test this technique with the paint you plan to spray with your airless handheld paint spray on test areas before commencing a final job.