My experience is that if a bottle of Tamiya paint gets old (as in several years), it sometimes gets grainy, and if so, brush-painting with it gets very difficult because your brush will keep lifting up semi-dried paint you just put down.Tamiya pints, at least those I've used, have their (minor) faults but I've not seen them described as grainy before.
For FULL Forum access you can upgrade your account here UPGRADE