2-pack primer fillers have a much higher solids content, which together with their particular type of resin base gives much better filling power plus less problems with sinkage than air dry products. Most are activated immediately before use, as once mixed their usable life is limited, depending on temperature and speed of activator used.
Please note that nearly all rely on isocyanate activators which have their own health and safety risks over and above normal spray-mist concerns.
Follow the manufacturers directions for activator and thinner quantities, as these can vary widely between brands, from 1:1 by volume up to 10:1 for some fillers. Application is normally one to three coats, depending on the surface build required. Allow adequate drying time between coats to prevent solvent build -up in the lower layers. Most two pack primer fillers are very thick and heavy even when activated and thinned, therefore a suitable spraygun set-up, i.e. large nozzle capacity, is needed to apply them at anything like a reasonable speed.
Some brands of two pack primers can, when activated and thinned appropriately, be applied as non-sand surfacers. This means simply that providing the previous preparation work has been carried out with care, and final flatting done with fine grade papers, the finishing coats of paint can be applied directly on to the primer without any further flatting. This method is normally only recommended for use in properly controlled spray booths to limit problems with dust, etc.
