How to thin exterior latex paint ?

Paints are divided into two categories: oil (or alkyd) and water-based paints. Oil-based paints can only be thinned or washed with petroleum-based or mineral-based products. In contrast, latex paints are water-based and can only be washed or diluted with water.

Water is the carrier of the solid products in latex paint. Successful dilution of latex paint requires a fairly accurate estimate of the viscosity of the paint and the addition of the correct amount of water.

What you will need
Equipment / tools
Cordless drill
Metal spiral mixer
Wooden stick for stirring paint
Latex or nitrile gloves
Measuring cup for liquids

  1. Acclimate the Paint:Before you thin the paint, acclimate the paint to ambient room temperature between 50 and 90 degrees Fahrenheit.
  2. Pre-mix the paint: Stir the paint thoroughly. If the paint has recently been shaken in the store, this step is not necessary. Over time, solids collect at the bottom of the can, making it difficult to determine the true consistency of the paint. Set the mixer attachment in the drill and mix the paint thoroughly. Run a wooden stirring stick over the paint to test it. If the stick is dragging on the bottom, it means the solids are not fully mixed yet. If this is the case, run the mixer in the jar until the paint is the same consistency from top to bottom.
  3. Assess Whether the Paint Needs Thinning. To fully atomize paint, paint sprayers require a paint that falls within a certain viscosity range. Consult the paint sprayer documentation for information about this range. 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.
  4. 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.
  5. Mix the water with the paint: Slowly pour water into the paint, stirring gently with a wooden paint stick. Check the viscosity with a cup or funnel before use.
  6. Test the paint on a similar surface: After the paint is diluted, stirred and left to stand (bubbles may form if you use the mixer vigorously), brush or roll the paint onto a surface similar to the one you are going to paint. If the paint is not thick 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. Also, keep this fact in mind when using multiple cans of the same-colored paint for a project.
  • 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.


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