gritty acoustics, other than replacing it I'd say not much can be done to clean it up
you could try making it dirtier that sometimes works, it's hard to remove what's already there if you are indeed looking for that "clean, transparent" acoustic sound
is it DI'd or mic'd??
if the grittiness is at a certain band of frequency then you could try & notch it out usinga parametric EQ on a tight Q setting
if possible though I would save yourself the headache & have it replaced