One benefit of working with latex paint is its ease of use. As soon as you bring the paint back from the store in mixed condition, latex paint is ready to go.
New, freshly mixed latex paint rarely needs to be thinned for most paint roller and brush applications. Paint manufacturers produce latex paint of a viscosity that works for an average number of applications. Since "average" does not apply to every situation, you may need to thin latex paint for applications such as paint sprayers or for older paint that has partially lost water content due to evaporation.
How Does Water Thin Latex Paint?
Paint is divided into two categories: oil-based (or alkyds) or water-based paints. Oil-based paints should be thinned or cleaned up only with petroleum or mineral-based products. In contrast, latex paint is water-based and should be cleaned up and thinned only with water.
Water is the carrier of solid products in latex paint. Successful latex paint thinning requires a reasonably precise assessment of paint viscosity and the addition of the correct quantity of water.
Equipment / Tools
- Cordless drill
- Metal spiral power mixer
- Wood paint stirring stick
- Latex or nitrile gloves
- Fluid measuring cup
- Viscosity cup (optional)
- Fine mesh cone strainer (optional)
- Latex paint
- Room-temperature water
Acclimate the Paint
Before you thin the paint, acclimate the paint to ambient room temperature between 50 and 90 degrees Fahrenheit.
Cold paint will flow more slowly and does not require thinning. If the paint is too cold, it may give a false impression that it needs thinning.
Pre-Mix the Paint
Mix the paint thoroughly. If the paint has recently been shaken at the store, this step is not necessary. Over time, solids tend to gather at the bottom of the can, making it more difficult to gauge the true consistency of the paint.
Chuck the power mixer attachment into the drill and mix the paint thoroughly. Run the wood stirring stick through the paint to test it. If the stick drags at the bottom, the solids have still not completely mixed through. If this is the case, run the power mixer in the can until the paint has the same consistency from top to bottom.
Assess Whether the Paint Needs Thinning
For precise calibration when using a paint sprayer, invest in a small low-cost viscosity cup. A viscosity cup is essentially a long-handled funnel with a calibrated hole at the bottom. Industry-standard viscosity cups include the Ford #4 Paint Viscosity Cup and the Zahn Immersion Viscosity Cup.
For a less precise but reasonably accurate way to gauge viscosity, dip some paint with a clean disposable cup and pour it through a kitchen funnel. Paint in need of thinning will clog the exit hole and either will not drain at all or will take an unreasonably long time to drain.
Strain Out Impurities
Solid paint chunks and film that have developed cannot be restored by thinning with water. Pull out large pieces with the hook end of a paint can opener. Remove remaining pieces and other impurities with a fine mesh cone strainer.
Measure the Water
To portion out the correct quantity of water, use a fluid measuring cup, not a dry measuring cup. The amount of water to add varies according to the current consistency of the paint and the consistency that you desire. Begin with 4 ounces of clean, room temperature water; you can always add more as needed. Paint manufacturers tend to cap the quantity of additive water at 8 ounces per gallon of latex paint for spray applications. Consult the paint can label or online paint specifications for your specific latex paint.
Mix the Water Into the Paint
Slowly pour the water into the paint while gently stirring the paint with the wood paint stick. Test with the viscosity cup or funnel before use.
Go slowly and be careful when adding water because this is only a one-way process: You can always add more water to paint to further thin it out but you cannot thicken the paint again. The best way to save paint that is too thin is to add it to a second can of paint.
Test the Paint on a Similar Surface
After the paint has been thinned, mixed, and left to settle (bubbles may develop when vigorously using a paint mixer), brush or roll out the paint on a surface similar to the one you intend to paint. If it isn't thin enough, add a small amount of water until it reaches the desired consistency.
Tips for Thinning Latex Paint
- Never use petroleum-based products to thin water-based latex paint. Any product that goes under the name mineral spirits or paint thinner likely is a petroleum-based solvent.
- If you want to eliminate brush or roller marks, consider incorporating a paint additive like Floetrol instead of water. For roller and brush applications, add 8 ounces of Floetrol.
- Adding water to paint dilutes the paint and thus lightens its color, so additional coats may be necessary to improve the color quality.
- One telltale sign that latex paint has lost water through evaporation is when the lid of the can is encrusted with paint. Even a pinhole-sized gap between the lid and the can is enough to allow water-based paint to evaporate over time.
- Combat evaporation by replacing the crusted lid with a tighter fitting plastic pour-and-store type of lid or by pouring the paint into a sealable paint can.