Is it always better to add the RETARDANT GEL to the specific amount of ink to be used or is it possible to just put it in the original jar and mix it in there? The reason for asking is I’d like to know if it will dry out or spoil the ink if it doesn’t get used for a long time.

The retardant gel that’s added to the ink doesn’t go off or go bad, so, in theory, you can add it all at once, up to a maximum of 3%.

Higher percentages would slow down the drying process too much, including on the mesh.
Having said that, since it isn’t always necessary to add gel, because it’s all dependent on the room temperature and the air humidity, we recommend adding gel only to the amount of ink you plan to use and only when necessary.

Any leftover ink, even if it contains gel, can be poured back into the jar.