Paints that are stored for a while may start hardening inside the bottle, as pigments deposit at the bottom.
If you use paints that come in a bottle you can open and have access to the paint (like Tamiya lacquers, Citadel, Humbrol, etc) an easy way to make them usable again is using an electric stirrer.
This simple tool will help you bring your paints back to life without having to spend minutes shaking them manually.
It works with two AA batteries (not included).
We have to prepare each shipment individually as they are ordered, so allow it in 3-4 weeks to arrive in the USA/Canada (to be safe) and 3-6 weeks rest of the world, though they are usually delivered much faster.