Small quantities of liquids can be measured using graduated cups and flasks – for liquids like paints and epoxies, measuring by weight is more accurate, wastes less material, and is way less messy. An inexpensive digital food/kitchen scale and a bit of math are all that’s needed. A five pound capacity is adequate, the scale should be able to display ounces as well as pounds and ounces, and should have a tare set button.
Example use: We want to accurately pour 1/2 quart from a full/new 1 quart can into an empty new 1 quart paint can. The procedure: Set scale to ounces mode, set tare to 0 (nothing on scale), weigh and record the weight of the full/new can of paint, say it is 51 ounces. Weigh and record the empty paint can, say it is 3 ounces. So weight of paint is 48 ounces (51 – 3 = 48). Put the empty can on scale, set tare to 0/zero (with empty can on scale). Watching the display, pour paint into the empty can until display reads 24 ounces. Now, each can has exactly 1/2 quart, 24 ounces by weight, 16 ounces by volume.
The directions on the paint can say to thin the first coat say 10% – this is by volume – so the thinner/reducer quantity would 1.6 ounces by volume. So you still need a small 1 ounce graduated cup for this, but the thinner isn’t messy, and you can weigh a volume of thinner for later reference if you want.
In conclusion: For this paint specifically, each ounce by volume weighs 1.5 ounces (24 / 16 = 1.5) by weight. Use this weight per fluid ounce value to measure by weight smaller quantities, and determine the weight per fluid ounce for other paint types you use. Thinner/reducer weighs significantly less than paint .