I'm Hello all, 

New here and hoping you can help. I'm puzzled as I've been doing this for 5 years with no changes and now suddenly having a big issue with this. 

i bake, cool, cut, fill and crumb coat. Then pop it in the fridge until it loses its stick then take it out and allow it to come back to room temp (remains unsticky to touch). I then do a final buttercream layer. fondant cover and it is completely smooth! 

later though I can see exactly where the buttercream is layered as it protrudes out giving an awful finish. I will try to attach a photo of a text cake or two. Sorry it's dark only one I can find that I didn't manage to cover up!! It's much more obvious in daylight lol 

to say it's driving me nuts is a understatement! I'd love to get this sorted before a very elegant four tier christening cake next weekend. 

Picture is rather dark...but even making it bigger, I can't see any flaws???

Yes, besides being dark and small your picture is sideways so we cannot see any flaws.  

Are you talking about the bulge that sometimes happens where the buttercream filling is between the layers?

MAgic moon you might be right.  I turned my laptop around and I think I can see that now that you mentioned it.

I have a tablet @kakeladi ‍ so I zoomed way in.  I might have cheated.

I fight this problem sometimes too @Lottiec90 ‍ and there are numerous posts on the subject that I've researched here. The first advice usually given is to be sure and make a dam of sturdy buttercream 1/4" from the outer edge before your filling is added. But since you've been doing this a long time I won't go any further with that. 

I did notice you refrigerate only a short time after crumb coat then return the cake to room temperature before adding the buttercream layer and fondant immediately after that.  I'm wondering why?  Meaning for me, the point of chilling the cake is to firm up the cake, buttercream filling and final layer so that it's as close to solid as I can get it before covering with the heavy fondant.  I also put the cake right back in the fridge after the fondant is applied to keep everything firm. 

There are a couple of things I learned researching CC forums that have helped me tons with the battle of the bulge.  As soon as I add the top cake layer over the filliing I press down gently on the top -mostly in the center.  The filling spreads to the edge and it also helps with those pesky air pockets for me. I chill after this then add a coat of ganache then chill again. I quit using buttercream for the outer coating altogether.  I live in a humid area that gets hot in the summer. The ganache is just sturdier and it gives me better control of the edges so they're crisp. Ganache also hardens to help keep the buttercream filling in place. I have also noticed that sometimes my fondant can seem heavier even though I feel like I've rolled it the same thickness as I always do.  Sometimes the weight causes a slight bulge despite my efforts to avoid it. 

I hope this helps a little and doesn't insult you by suggesting things you already know!