Signs of damage: If the paint has gone bad, it will look curdled like cottage cheese or overly thick like a pudding. If you dig out an old can of paint and you have either of these, it's best not to use the paint and dispose of it properly.
Freezing temperatures can do permanent damage to the emulsion in paint, causing the paint to become a strange consistency. Paint that has frozen and thawed may become ropey, stringy or clumpy. It may be the consistency of cottage cheese or gritty, like sandy water.
If the paint feels lumpy, strain it before working on your project. You can also add somewater or a paint thinner to restore its consistency. You can use a pair of scissors to remove any lumps while the paint is still dry. After the paint has dried, you can smooth out the canvas using sandpaper.
Is it lumpy? Paint should be creamy and smooth. If you stir it and it's full of lumps -- it's bad -- get rid of it.
Throw it away. If it is mold there is more in there than you can see. You do not want to risk bringing that mold into your home or workspace.
You might notice the consistency of your paint has changed. There may be a “skin” on the surface or lumps that won't stir out. If either of these is present, your paint has most likely expired.
Solutions for Painted Surfaces with Mildew
Start by washing the surface with a solution of one part liquid bleach and three parts water. Apply the solution and scrub the mildewed area. Allow the solution to remain on the surface for 10 minutes. Rinse thoroughly with water and allow the surface to dry before painting.
What happens if you use expired paint? If the paint has a bad smell, you'll be adding that odor to your walls. In addition, expired paint will not provide even color coverage, the finish may not be smooth, and the paint is more likely to crack or chip.
Make dried paints liquid again with water
As acrylic paints are water-based, you can make these paints liquid again by adding water. Hot water is best for this, which you pour drop by drop into the pot. Then stir the dried paint thoroughly and wait for the water and dried acrylic paint to mix thoroughly.
Scrape or Sand the Drip
A paint drip creates a raised area highlighted by the paint's shininess. Therefore, the first step after the paint has dried is to cut down or sand the raised area with an appropriate grit sandpaper. Start lightly scraping down the drip with a clean scraper, razor blade, or 5-in-1 tool.
Whipping cottage cheese is the easiest way to remove any textural issues — the lumps disappear, and you're left with a versatile light and airy product. Simply use your food processor or high-speed blender to pulse the mixture until it's smooth. You can always add a little olive oil for an even fluffier texture.
As theacrylic paint stays exposed to air, it starts losing the moisture stored inside it, which makes the pain a little lumpy and stiff. The best way to fix this is to replenish the lost moisture by adding some water.
Most paints last one to two years in an open can. Latex paints can last an average of 10 years in an unopened can. Oil-based paint lasts up to 15 years in an unopened can. The paint goes bad by hardening or becoming rancid when air or debris gets into a can that isn't sealed well.
I coated an entire smelly acrylic painting with bleach and let it sit. Now, the smell is gone! I've painted over it with new paint and all seems fine. This solution saved me for now, but my advice is to take a sniff test before you work with old paint.
Restoring antique single stage paints can either be done the caveman way, using an aggressive compound and risking grinding all the thin paint off the car, or it can be done the careful way, and that is FIRST conditioning the paint using the unique, time-proven rich polishing oils found in the #7 Show Car Glaze and ...
Immediately wipe off fresh splatters. For those that have dried, sand down the drip or spill until the surface is smooth. Then clean off the area with a damp cloth and repaint if needed. If you need primer, prime first and wait for it to completely dry before you paint.
You don't want your paint to be lumpy, but lumps don't mean it's unusable. You can strain the lumps out using a simple paint strainer and be good to go in no time. If it's still too thick after straining try adding some water to latex paints and minerals spirits to oil paints to thin back out.
Paint Shake and Re-Tints
If you bring your old paint to a Home Depot paint center, you can have it shaken up in their machines. This saves the effort of stirring up your old paint, but it also sets up another great service: paint re-tints.
Anything that looks lumpy, gives off a sour smell, has signs of mold, mildew, or rust, or has a thick skin under the lid is a good indicator that your paint is expired. “Lid skinning is much like pudding where the paint gets a thick skin on top or on the lid that is hardened,” says Hill.
When mold has been painted over, it can start to show through the paint. It often looks like stains on the wall that take on a yellow or brown color. You'll likely notice these stains through light-colored paint or on the ceiling.
Signs of damage:
If it smells unpleasant or you see mold, you will need to discard it (correctly). And, yes, mold can grow inside a paint can and be a serious health hazard if you put it up on your walls.