With the cost of paint materials rising all the time, it only makes sense to try and do everything you can to avoid wasting any of it. Every little bit you can save can be applied elsewhere. Mixing paint in separate containers is just one example of how paint gets wasted, as some of it is always left behind.
3M has found a solution by developing a closed system that allows the paint to be mixed, strained, and sprayed using a unique cup and disposable liner. The added bonus of the paint-preparation system mini cups (available in 6 oz, 650 ml, or 850 ml) is that they allow the spray gun to function at any angle to reach hard-to-get areas; just try spraying upside down with a traditional spray gun. If you already have a siphon-fed or gravity-fed gun, 3M has adapters to mate the mini cups to most of them.

Once the existing cup is removed...

Once the existing cup is removed from the spray gun, the adapter is installed to mate the paint preparation system (PPS) to virtually any gravity- or siphon-fed spray gun.

Before the disposable cup...

Before the disposable cup (right) is placed into the cup, a 3M Mix Ratio Film can be inserted to mix the paint to manufacturer's guidelines. Depending on which way you insert it, it can be read from either the inside or outside. An alternative is to use a standard mix-ratio stick inserted between the cup and liner.

Once the paint is mixed as...

Once the paint is mixed as needed, a lid (with built-in filter) is snapped onto the top of the cup/liner. The locking collar is then placed onto the cup/liner and turned until tight.

Although you're most likely...

Although you're most likely not going to have a mechanical paint shaker at home, 3M does provide a cup shaker core to distribute the clamping force across the lid when shaker machines are used.

The cup with the mixed paint...

The cup with the mixed paint is then snapped onto the gun with the appropriate adapter, making sure the lid fingers are fully engaged above the adapter edge.

The 3M PPS can be used like...

The 3M PPS can be used like your traditional gravity-fed spray gun cup, but all you need to do is invert the gun (with the air attached) and pull the trigger (full open) to bleed the air out of the cup if you want to be able to hold the gun at any angle. The liner will collapse, pushing the paint toward the gun.

Here you can see the system...

Here you can see the system working with a siphon-fed gun, which can now be used in any position as well.

When you're done painting,...

When you're done painting, the lid and liner can easily be removed and disposed of properly.

You can disconnect the air...

You can disconnect the air line, invert the gun, and pull the trigger (full open) to let the air bleed back into the cup if you're not done painting and need to add some more paint to the cup.

If there's paint in the cup...

If there's paint in the cup and you need to mix a little more material, you can insert a paint stick between the cup and liner with the bottom at the top of the paint and add accordingly.

Cleaning the gun and adapter...

Cleaning the gun and adapter can be done easily using a lot less solvent-up to 70 percent less, according to 3M.