Going over a surface immediately after airless spray painting with a paint roller is referred to as back rolling. There are a few reasons why back rolling with a paint roller is required in certain applications.
Applying paint with an airless spray gun to coarse or porous surfaces such as previously unpainted rough sawn timber, new cement render or previously unpainted masonry such as bricks concrete blocks and other similar surfaces will result in any depressed indentations receiving very little or even no paint. Continuing to apply paint with an airless spray gun in these cases will accentuate the effect by applying more paint around the indentations further restricting any sprayed paint from penetrating into the indentations.
Brand new plasterboard will have a surface texture variation between the actual plasterboard and the filled plastered joints. Further texture variation can occur when the sanded plasterboard joints and the adjacent face of the plasterboard is also sanded. Painting new plasterboard with a paint roller applies texture to the paint finish which tends to camouflage these variations in surface texture. Simply applying the paint with airless spray gun does not apply any paint roller texture and variation in surface texture between the plasterboard surface, the sanded plasterboard joint, and the sanded plasterboard paper surface adjacent to joints becomes very noticeable especially where there is any gloss level in the paint.
Back rolling with a paint roller immediately after applying the paint with airless spray gun will overcome many these effects. Back rolling needs to be done immediately after applying paint to the surface and is usually easiest if one person is applying paint using the airless spray gun and another person is following behind with the paint roller. Paint roller should be loaded with paint without being overloaded. Applying paint using an airless spray machine and back rolling is much faster than using just paint roller and coating penetration is enhanced plus overall painted surface finish will appear much more even.
If a second operator is not available to backroll immediately after applying the paint with airless spray gun attaching a power paint roller accessory to an airless spray unit may be an alternative.