Not entirely true. You can add the dye to the grainfiller, which is what Gibson did to the Cherry Red SG's. If you use water based grain filler you can use any type of dye, and it will take. If you use oil based grain filler you have to use a solvent based (laquer thinner, denatured alcohol) dye. You are dying the wood and the grain filler. I have to go back and look, but I think Gibson dyed the grainfiller, applied a sealer coat, applied a toner coat and then cleared. If you get the Finishing, Step by Step book that Stewart MacDonald sells, they have the recipe that Gibson used on the original SG's and V's. If it in not exact it is extremely close. But they do say, one of the keys to getting the color right is the dyed grain filler.
Thanks, this is very helpful. I will have to get that book (I'm getting quite a stack of them!) for when I graduate to sprayed-on finishes, which I'm planning to do on my next guitar.
My grain filler is water-based (it's ColorTone, like my cherry red dye), so I could dye it, no problem. But after that I'm going on to a soft-looking oil finish, rather than spraying anything. Do the other steps (sealer and toner) have any application to my tung-oiled project? I'm thinking that after the dyed filler, my next (and only) steps would be to rub in about 8-12 applications of wiping varnish over two weeks, wet-sanding the last ones. I admit to being clueless, though, so feel free to correct me.