Use special solvents: there are special solvents for oil paints that help restore a dried-out tube. Simply apply a small amount of solvent to the top of the tube and leave it for a while. Then, gently mix the paint with the solvent using a knife or brush.
Baby oil was championed for years as an excellent option to put the oil and suppleness back into the art. The instructions were to use a soft cotton ball coated with baby oil and gently rub it in a circular fashion over the painting. This initially makes it shiny and the colors look bright.
I would crush up the dried paint into a half or full pan, add a few drops of water, then let it mellow. The paint probably won't become a consistent paste again but it should soften up enough to be able to pack it down into the pan as the water evaporates.
You could try laying down some paper towels (something absorbent like viva or blue shop towels) and apply firm yet gentle pressure to draw out some excess oil and help create a thinner paint film, taking care to not actually move the paper towel around so you're not smearing wet paint around.
Linseed Oil: adding a small amount of linseed oil can revive dried paints. However, be careful and add the oil gradually to avoid making the paint too liquid. Plasticizers: some artists prefer to use special additives that make oil paints more pliable.
You will want to dry out the remnant paint before disposal. You can use cat litter, sawdust or concrete mix to speed up the process. You can also just leave the lid off for a few days. Never dispose of oil based paint by pouring it on the ground.
DIY Solutions for Dry Paint
One popular method is using a mixture of vinegar and water for acrylic paints that have started to dry but are not yet completely solid. The acidity in vinegar can help break down the paint, making it easier to mix back into a usable consistency.
If your paint has separated into semi-transparent and opaque liquids, try stirring it to see if it comes back together. Wait a few minutes. If your paint reseparates quickly, you'll know it's no longer good to use because the solvent and pigment won't work together to properly stick to the wall.
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 ...
If kept properly, your precious tubes of oil paint can even last you decades. In fact, oil paints can have a shelf life of 15 to up to 40 years! That's why seasoned artists have grown to love this medium for painting!
Methods to store oil paints when you are not actively using:
hang them upside down using a binder clip at the end of the tube and stick them to a board (this lets the lighter weight oil to flow up and he heavier pigment to flow down to the cap)
For dull spots, rather than whole areas of dullness, you can apply a small amount of Artists' Painting Medium on a clean cloth. Rub it gently into sunken areas, wipe off any residue and leave it to dry for a day or two.
Repainting over an old dried oil painting involves first cleaning the surface thoroughly to remove dust and dirt. If the painting is cracked, you may need to apply a layer of gesso to create a smooth surface. Use a light hand when applying new paint, and allow proper drying time between layers.
Drying oils solidify and harden through chemical reactions with oxygen, entailing two main paths—oxidation and crosslinking [1]. All paints show a rapid initial oxygen uptake followed by mass loss due to the decomposition of oxygenated compounds with low-molecular-weight molecules lost by evaporation [2].
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.
Oil based paints have a shelf life of anywhere from 2 to 15 years. Depending how well it was sealed and stored. One of the best ways to tell if your stored paint has gone bad is to test it on a piece of board or timber. If lumping occurs it could be that chunky sized bits of paint have formed despite a vigorous stir.
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.
Activating a new paint
New paints may need a little more water to activate. You may either spritz it with water or work the paint with a wet brush.
You can try heating up the room but most of the time you will just have to be patient. Alternatively, you can strip the paint and try again.
To solidify paint, use one of the following methods:
Mix latex paint with an equal amount of cat litter, stir in completely and allow to dry. Mix latex paint with a latex paint hardener; stir and allow to harden. Paint hardener can be purchased at your local hardware store. Follow the directions on the package.
Kitty litter is recommended for hardening oil-based paint. Once it is solid, place the can next to your trash with the lid off so your garbage collector can see that the paint is dry. Since garbage collectors cannot pick up liquids, lids must be left off so they can see that the paint has been solidified.