Do I use a crusting buttercream to do a crumb coat or not. I am putting a fondant over top.

I would use regular buttercream for that...because it sticks to the fondant and seals it onto the cake. I'm not certain if crusting buttercream would do that?

I just use whatever I have on hand..both seemed to work just fine!

I use a crusting type of BC and it works fine for me with my crumb coats

i never use a crumb coat when i ice... i think its coz i ice cakes alll the time at work, that im just used to icing with no crumbs... but when i use fondant, i give it a normal layer of buttercream and just make sure its cold and hard before i put the fondant on