I know this has been discussed before, but would like see if there are more people out there that have tried this. I have Toba Garrets book, but she doesn't give the ratio of cake to buttercream and filling. Its very general, does anyone have a tried and true ratio for this. I have a cake that stuck and has some holes, so I thought this would work. Toba uses it for under fondant, so that there are no bulges or sagging, just wonder how it works for this. I haven't covered cakes in fondant before, always done buttercream, so i'm really worried about it. Its just that I have heard people talk about how the fondant looked perfect, and then sagging or air hole bulges occur, I want it to look perfect, its an all white cake, with minimal decorations so was wondering if this method will prevent those problems from occuring, thanks

ok, so to fill any cracks or holes or whatever you can still use buttercream or ganache. you'd want a layer anyway to go under your fondant anyway to crumb coat it.
Before laying your fondant you want to have a nice smooth base to work from, my preference is the Planet Cake method of gnanching and hot-knifing. Now, I'm in no way an expert but my advice would be not to roll the fondant out too thinly if you want a perfect finish, maybe about 1/4 inch or so and then use a smoother to push out any air bubbles, if there are still air bubbles, use a pin to let the air out then smooth again. you have to work quickly so that the fondant doesn't dry and you should secure the edges so the fondant doesn't stretch. there's plenty of tutorials online. Good luck with it.

When I spackle, I start with half cake scraps and half buttercream. Then I add fruit filling, a little at a time until the spackle is loose enough to work with. There's no definite proportion because fillings, buttercreams and cake scraps come in a variety of textures. So it depends on what recipes you're using. You just have to get a feel for it.

I find that it makes a very good base for fondant. Depending on the type of cake I'm doing, I've used spackle as a crumb coat/crack filler, with buttercream over that, or as a crumb coat with more spackle over that as the final frosting under the fondant.

I also use spackle as a filling on occasion. When I torte two 2" cakes so I have four layers of cake with three layers of filling, I often use spackle for the center fillings with fruit filling for the top and bottom filling layers.