Most water-based versions of KILZ-brand primers may be thinned by adding a small amount of water, including KILZ Latex Primer, KILZ Premium Primer and KILZ Max Primer. However, KILZ advises against thinning its oil-based primers, including KILZ Original Primer and KILZ Odorless Primer.
Thinned primer is helpful when using a spray-application method, since thick primers may clog the spray nozzle. Thinned primer also covers more surface area, reducing the cost of materials for large projects. When thinning water-based primers, KILZ recommends using no more than one-half pint of water per gallon of primer. To thin, pour the needed amount of primer into a container, then add water and stir until completely mixed. It is important to note that thinned primer is less effective at blocking stains. Before using thinned primer on large surfaces, apply the mixture to a small area, preferably a spot with stains, to ensure that the thinned primer is effectively covering any stains. Though many homeowners prefer water-based primers for their easy application and clean-up, the repair experts at the television show "This Old House" recommend oil-based primers, sometimes called alkyd primers, for their superior penetration and stain-blocking results. KILZ recommends against thinning oil-based primers, though some professional painters do use mineral spirits to thin them.