On some T-shirts I’ve had problems with the white Texprint water-based ink (printed on dark material) not being particularly resistant to washing.
The problem only occurred on some pieces, seemingly at random, so I’m struggling to understand what the issue might be. Could it be that the ink was too old (6 months)?
The problem is not the age of the ink.
It’s more likely to be down to variations in the quality of the cotton. Some fabrics from Asia are surface-treated to improve their look, but this can have an impact on how well the ink adheres to the material.
To increase the probability of the ink being wash resistant, my suggestion is to always use heat drying, even if, as is the case with Texprint Mono, the standard approach is to use cold drying.
And, in addition, I’d add 5% of the Texprint universal catalyst.
To heat dry Texprint ink, even a standard iron would be better than nothing.