I am making a barbie cake and when I crumb coated it last night, I used BC that was too watery. Now the icing didn't dry and I scrapped it off. The cake feels moist and slightly slick from the bc. I am afraid the fondant will slide off with bc. Is royal icing a better choice for an undercoat?
Typically, just a crumb coat of buttercream is useful to smooth the surface and allow the fondant to stick. Unless the fondant is very heavy it shouldn't slide off.
It just sounds like you need to add PS and perhaps a little merengue powder to your BC--I even add flour to mine. It should pretty darn thick.
RI would dry hard, not taste very good, and the fondant won't stick to it without a spritz of water.
Some bakeries just brush on warmed jam before applying fondant, but I prefer a good BC.
I crumb coat mine, refrigerate, then apply another coat of BC while the cake is still cold and the crumb coat is still hard. The second coat always goes on nice and smooth.