Switching from one sprayed material to another does not always mean stopping to do a complete cleaning sequence. More often than not, I switch from one finishing material to another simply by spraying a succession of solvents through the gun before I reload with a different, incompatible coating.
Let’s say you just finished spraying waterbased polyurethane, and now need to spray oil-based polyurethane. Logic would suggest that you could rinse out the gun by spraying a few ounces of water through it, followed by a few ounces of mineral spirits, the solvent for oil-based polyurethane. Unfortunately, that won’t work. The mineral spirits would not remove all the water, and you might end up with water droplets in your finish.
Miscible Solvents
The key is to remove all traces of each solvent by "chasing" it with a succession of fully miscible solvents. The term miscible describes two liquids that will mix in any proportion. Alcohol and water are miscible because you can mix any amount of either one into any amount of the other. Water and mineral spirits are not miscible, so water won’t chase out mineral spirits, nor vice versa. By using a succession of miscible solvents, you can remove all of the previous solvent at each step. The photo will give you a guide to which steps are necessary. For example, to go from waterbased polyurethane to SealCoat™ (dewaxed shellac), you need only rinse with water, then denatured alcohol. But to go from waterbased polyurethane to oil-based polyurethane, you’d rinse with water, followed by alcohol, then lacquer thinner, and finally mineral spirits.
To go the other direction, follow the steps in reverse order.
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| Each solvent in the photo above is miscible only with the solvent directly next to it. Start the rinse process with the solvent for the finish being removed, and end with the solvent for the finish being loaded. Just be sure to use all the solvents in between — don’t skip a step. |
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