I was asked that same question by Gary a week or so ago and I gave him the following answer:
Although I haven't used Rit dye in particular, there are some things that you may want to think about . . .
As far as the Rit goes, I guess there would be two things that I would be concerned with: fully dissolving the colors and the amount of water needed for application. Now you can probably use the liquid so you don't have any grains left. I found the powder dye to be like Jell-o whereas you can sometimes see little grains in it even after stirring, stirring and more stirring. The water is another issue.
I found that using a hard, tight wood such as maple takes better to water better than something like poplar or pine. You will get far less raising of the grain because the molecules are packed much denser and less water is allowed to penetrate. But that also means that less pigment will penetrate, or it may penetrate unevenly. For this reason I would keep scraps of wood handy and test everything.
I realize some people recommend sealer to seal the grain first. However, if you seal the grain, how do you expect the stain to penetrate and not just sit on the top?
I am hoping that others who read this and have used Rit as a dye would share their results and help answer these questions.
Thanks in advance, Sheila