What Happens if Paint is not stirred?

Question

I just bought a 1 gallon of Sherwin Williams, pure White Shadow Solid color paint. The customer service made it custom on spot. When painting touch ups around the house, I realized I forgot to stir the paint with the wooden stick, before applying. I did not see any issue, and the paint looks fine after two days. I painted on the same day it was manufactured.

1. What are the consequences of not stirring pure paint? Will the paint peel off easier on my wall? Will it fade off? Does it emit harsher emissions (although the paint is low VOC)?

2. More importantly, Does Sherwin Williams already stir the paint when they make it on the spot? How long does the effects of stirring paint last before I need to restir? I painted 5 hours after it was made.

Note: I did not see an answer online, only about stirring mixture of different color paints, but this is pure single solid color paint.

Curious if I should reapply the touchups around the house, I already disposed the paint few days ago.

Answer 1

If you leave a bucket of paint in storage for a few weeks/months, some of the components will float to the top, and others will sink to the bottom. Often the solvent will rise to the top, and the pigment will sink. That's pretty noticeable, basically you get a layer of clear liquid in the pot, on top of the paint. In this case, you must stir it to mix everything properly before using it.

However, if you bought a custom color that was made to order in the shop, then there's no problem. The machine that mixes in the colors already stirred the pot pretty vigorously, and 5 hours definitely isn't enough to cause trouble.

Answer 2

Separation starts immediately and proceeds at its own pace. On the common the paints they sell for painting houses, I would expect separation to be very slow, and not be significant after only 5 hours.

What is happening is the resin and pigments (heavy) are settling toward the bottom while solvent (stuff that vaporizes away) settles toward the top. So your paint will be a little bit scant on pigment, but not enough to matter, I wouldn't think.

As a counterexample, "cold galvanizing paint" has resin that is 90% zinc metal, and it's heavy and starts separating in seconds! Constant stirring in the pot is essential or you finish the job with a huge glob of zinc peanut butter in the bottom of the pot, and what you painted was mostly thinner lol.

Another thing that happens is the various pigments weigh different weights, and will separate from each other given a long time (months or years). But you seem like a person who doesn't keep paint, so that's fine.

Modern paint (read: for houses) is almost all "latex" and has very low VOCs (toxic solvents that cause smog). Since it's less toxic, it doesn't kill mold and bacteria (whoops!) Using it creates a rancid room that sickens people and is extremely difficult to seal - usually best answer is smash out the drywall. So "not keeping paint" is reasonable.

You might want to paint some sample sheets (cardboard primed with the same primer) and keep them on file for if you want to match the color again. Custom color mixing is not perfect.

---

Note that paint should not be dumped down the toilet where it damages the sewage treatment process, or thrown in the trash where the can will burst open, making a mess for garbage handlers (I've seen more than one line down the street from cans burst open inside the garbage truck). If it makes it to the landfill, it will leach into ground water. It's non-toxic, but not that non-toxic.

Dispose of paint according to local government advice.