TL;DR
Dry out latex paint by mixing in paint hardener or cat litter, letting it set until it is solid all the way through, then putting the open can in the trash with the lid off. Only water-based latex can be dried and trashed. Oil-based paint never can.
Half-used cans stack up in every garage. The good news is that latex is the easy case, and one can of hardener handles several gallons.
First, Find Out What Kind of Paint You Have
Read the label before you do anything else. Latex, acrylic latex, and anything that says "clean up with soap and water" is water based. Oil, alkyd, enamel, varnish, shellac, lacquer, and anything that says "clean up with mineral spirits" is solvent based and is handled as hazardous waste.
If the label is gone, run the alcohol test. Put a little rubbing alcohol on a rag and rub a dried spot of the paint. Water-based paint will soften and come off on the rag. Oil-based paint will not.
Solvent-based products belong in the hazardous stream along with the items in what qualifies as hazardous waste. Do not try to dry them out.
Why Wet Paint Is Not Allowed in the Trash
Garbage trucks compact their loads. A can of liquid paint bursts under the ram, coats the inside of the hopper, and runs out of the truck body onto the street. It also soaks into everything else in the load and ends up as free liquid in a landfill cell, which is exactly what landfill operating permits are written to prevent.
That is why the rule everywhere is the same: solid, not liquid. Once latex paint is fully cured, it is an inert solid and stops being a problem.
Local rules still vary on whether they want the lid off, the can open, or the dried paint removed from the can entirely. Check your city or county solid waste page for the specific requirement before collection day.
Method One: Paint Hardener
Paint hardener is a polymer powder sold at hardware and paint stores in small packets. One packet typically treats a gallon or less, and the instructions on the packet are the ones to follow.
Work outdoors or in a garage with the door open. Open the can, pour in the hardener, and stir it thoroughly with a stick until the powder is fully worked in. Leave the can open and out of the rain.
Most cans firm up within an hour and are fully solid in a few hours. Poke the center with a stick before you set it out. If the middle is still wet, add more hardener and wait longer.
Method Two: Cat Litter or an Absorbent
Clay-based cat litter does the same job for less money, and it works well on cans that are less than half full.
Pour litter into the paint at roughly a one-to-one volume with the remaining paint, stir it in, and add more until the mixture stops slumping. Sawdust and shredded newspaper also work, though they take longer and are messier.
For a can with only a skim of paint left at the bottom, you do not need an additive at all. Just pull the lid off and set the can somewhere dry and out of reach. A thin layer air-dries in a day or two.
For a full or nearly full gallon, do not try to dry it in the can. Pour it into a cardboard box lined with a plastic bag and filled with litter, in shallow layers. A deep pool of paint skins over on top and stays liquid underneath for weeks.
How to Tell When It Is Actually Done
The test is mechanical, not visual. Push a stir stick down through the center to the bottom of the can and pull it back out. If any liquid follows the stick, it is not done.
Tip the can sideways over a scrap of cardboard for ten seconds. Nothing should move. The surface should be rubbery to firm, not tacky enough to string.
Set the can out with the lid off so the crew can see the paint is solid. A closed can gets left behind, because from the outside there is no way to tell.
Cold, damp weather roughly doubles cure time. If your garage sits below 50 degrees F, expect to wait.
What Still Has to Go to a Drop-Off
Drying works for latex only, and only for small to moderate volumes. Several things stay outside the method:
- Oil-based paint, stain, varnish, shellac, and lacquer
- Paint thinner, mineral spirits, and brush cleaner
- Aerosol spray paint cans, full or partial
- Anything you cannot identify
- Large volumes, meaning many gallons at once
Many states run paint stewardship programs where retail paint stores accept leftover architectural paint for free, including latex in good condition. Availability and the accepted quantity vary by state and by store, so call the location before you load the car. Your options are laid out more fully in what to do with old paint.
Paint scraped from a home built before 1978 is a separate issue entirely, covered in lead paint debris.
FAQ
Can I pour latex paint down the drain?
No. Even though latex is water based, the solids clog plumbing, coat sewer lines, and add pigment and biocides to wastewater that treatment plants are not designed to remove. The same goes for pouring it on the ground or into a storm drain.
How much hardener do I need for a full gallon?
Read the packet, because concentration varies by brand. As a rule, one standard packet handles up to about a gallon of paint, and a can that is more than half full often needs a second packet. Adding too much is harmless. Adding too little means waiting and re-dosing.
Is the empty can recyclable after the paint dries?
Sometimes. A clean, fully dry metal paint can is scrap steel, and some transfer stations and scrap yards accept them. Many curbside programs do not, because they cannot tell from the truck whether the can is dry. Check locally, and never put a can with wet paint in a recycling bin.
Can I still use paint that has been sitting for years?
Open it and stir. Latex that has been through a freeze cycle will look curdled or stringy and cannot be saved. Paint that stirs back to a smooth consistency and does not smell sour is usually fine, though the color may have shifted slightly from the original batch.
What do I do with dozens of cans from an estate or a shop?
That volume is worth handling as a single project rather than one can at a time. Sort latex from solvent based first, then take the solvent products to a county hazardous waste event and either dry or return the latex. Most county transfer stations run a household hazardous waste day, so check the schedule before you start.
Clearing the Rest of the Garage
Paint is usually the last thing standing between a cluttered garage and a usable one, and it is the one item a hauler cannot simply load. Once the cans are dry or dropped off, see junk removal for the rest of the garage. Take a look at getting rid of old paint cans for the disposal side, then get a quote for the rest.
