I have been makeing fondant covered cakes for 2 years and buttercream cakes for 14 years. But just in the last few months I have been having the worst time with air bubbles both on my fondant covered cakes and buttercream only cakes. The only change is that now I bake in a commercial kitchen instead of my own. I bake on Wednesday, fill and cover in buttercream on Thursday and cover in fondant Friday and decorate. I put the cakes in fridge after they have cooled wrapped in plastic wrap. I don't know if it matters but some of the buttercreams I use are half crisco and half butter and others are just crisco or just butter. It happens with all of them. Sometimes the bubbles appear quickly and sometimes it happens after the cakes are completey decorated and in route to the event. PLease can anyone add any insight to the problem of air bubbles. I read a few different forums and haven't found an answer.
I have found that air bubbles are due to how high of a speed you whip your ingredients. You say you let your items REST for a day so I may suggest using a lower speed during the whipping progress.. Or let them rest an extra day. Any other suggestions.. I would like to hear.
if your cake is not at room temp when you decorate, that can cause air bubbles. i think it can happen when your fondant isnt properly stuck to the crumb coat either.
Not 100% sure what kind of bubbles you are talking about, but if it's blow out type (that come out the side), add a few pin holes in the side of the cake. This will allow air to come out if it's trapped. I used to get blow outs.... not any more