The WB.5.125 Latex Viscosity Cup

Throughout my experimenting with spraying latex I refer to thinning the paint with about 20 to 25% water or windshield washer fluid. After a while you kind of get a feel for how much to thin the paint to get it to spray well, but that feel can be hard to explain to someone else. The professional automotive painters use a viscosity cup to measure how thick the paint is. A viscosity cup is a specially designed ladle with a precisely dimensioned hole on the bottom. The cup is dipped into the paint and the paint is allowed to run out through the hole. Viscosity is measured by the amount of time that it takes cup to empty. The problem is that these cups are designed for very thin automotive paint, not latex. As a solution I've designed the WB.5.125 Latex Viscosity Cup. The nomenclature for the WB.5.125 Latex Viscosity Cup is crucial to it's understanding. “WB” stands for the Water Bottle from which the WB.5.125 Latex Viscosity Cup is made. “.5” indicates the cup capacity of ½ cup. And “.125” is the size of the 1/8” hole drilled in the cap to precisely meter out the paint. During the development of the WB.5.125 Latex Viscosity Cup I experimented with different size cups and orifices, but settled on the ones described here.

WB.5.125 Latex Viscosity Cup Construction

You need to start with a water bottle. I prefer the 16.9 oz size because it leaves enough material for the handle. These are available individually of by the case, and they come full of water. Drink the water from a bottle so as to empty it. Then, using a ½ cup measuring cup and a small funnel, pour ½ cup of water into the bottle. This does not have to be the same water that you removed from the bottle. Any water will do. Cap the bottle and turn it upside down. Use a Sharpie (product endorsement) and draw a line around the bottle indicating the ½ cup water level. Remove the cap and drink the water. Using scissors (they are more controllable than a knife) cut along the Sharpie (product endorsement) line half way around the bottle, then cut vertically to create a dipping handle. Now for the .125 metering orifice. Using a 1/8” drill bit make a hole in the center of the bottle cap. Clean any plastic burrs from the edges of the hole with a knife or by twisting a larger drill bit with your fingers. Be careful not to make the hole bigger than 1/8”.

Calibrating the WB.5.125 Latex Viscosity Cup

Dip the WB.5.125 Latex Viscosity Cup into water and time how long it takes the cup to empty. It should be close to 12 seconds. Is it is longer, try cleaning up the metering orifice with a 1/8” drill bit, twisted between your fingers. If the water flows out faster than 12 seconds you will need to remanufacture the metering orifice from another cap.

Using the WB.5.125 Latex Viscosity Cup

Mix your paint in a clean container that is large enough to allow you to use the WB.5.125 Latex Viscosity Cup. Dip the WB.5.125 Latex Viscosity Cup into the paint, pull it up, and note how long it takes to empty. I find that about 24 seconds works for me for both spraying the color, and for brushing the primer. I stop timing when there is still a little paint in the lid but the flow has slowed significantly. The viscosity of the paint will make those last fractions of an ounce come out very slowly so I stop timing without waiting for the cup to totally empty. You will see what I mean when you try it. I have tried timing un-thinned latex paint, but I gave up after 5 minutes of waiting for the cup to empty.

So there it is. The WB.5.125 Latex Viscosity Cup is an affordable, available, easy to manufacture, device that will allow you to get repeatable results when painting. Plus, it's healthy since you just drank 16.9 oz plus ½ cup of water!