Tack rags are used just before painting to remove the lightest dust or dirt from what would otherwise be considered a clean, ready to paint surface.
Tack rags are used before final primer and topcoat applications. They are usually not needed when applying fairing materials or surfacers.
When tacking a primer or topcoat surface, use only Deluxe Tack Rags (#73009). They have the proper type and amount of resin to pick up dust without leaving contamination on the surface. Other tack rags can easily leave a residue that causes fisheyes and cratering of the topcoat.
Be careful not to use excessive hand pressure when using tack rags. A light dusting is all that is necessary, let the resin do the work. Even Deluxe Tack Rags can leave a resin on the surface if used improperly.