This has happened to me twice now- I made an R2D2 cake for my son's birthday. It was two 6" cakes on top of each other, about 10" tall. I covered it with Satin ice fondant and it was perfect. Came back to it 30 min later and bubbles were forming - it was humid so I thought it was that, put pin holes in them and tried to flatten. Didn't work. It started cracking, then started rippling. I'll try to post a pic. I made a 'Plex' (from Yo Gabba Gabba') cake last year (same size body) and the same rippling happened. I made sure this year not to make the fondant too thin, but it still rippled.
I need to know what I did wrong so this doesn't happen again. Any ideas?

Did you cover it with the fondant when it was cold? Sometimes the cold traps air and it tries to escape. That forms the bubble.

It looks to me like it was rippling around the bottom half, am I right?

Did you let the cake settle before you put the fondant on? Were there adequate supports for the top half?

It almost looks to me, without knowing anything about how it was constructed, like the bottom was squishing a bit from weight and causing the fondant to ripple. I may be wrong, but it's worth a thought.

Another thing that contributs to bubbles (aka:blow-outs) is not adhearing the fondant enough to the undercoating (be it b'cream, ganash - whatever used). You need to rub, rub, rub that fondant so it touches the base undercoating.
I found a fondant smother works best. For yrs I refused to get one - others had said the hand was enough - but once I aquired a smoother I saw a vast improvement in the look of my finished fondant covered cakes

Thanks for all this info, I REALLY appreciate all your expertise. Yes, most of the bubbling was on top, and the rippling was where the two cakes join, and on the bottom half.
I did cover the cake when it was cold, so that must account for the bubbles (I had no idea, self-taught, no-one to ask!). I used a meringue buttercream and once I crumb coated it I put it in the fridge overnight. I covered it the next morning, so I guess it was too cold. But wouldn't that be enough time for it to settle? I put 5 wooden dowels (the narrow ones wilton sells) in the bottom tier of the cake, and 5 more in the middle tier (to support the head). Should I have used more for a 6" cake, or maybe plastic ones instead?
I used my hands to adhere the fondant - it was humid so I was afraid the smoother would stick to the fondant - but I'll be using them from now on!

You did a great job otherwise. Looks great

First, I'll throw a wrench in things by saying I *ALWAYS* chill my cakes before applying fondant.

It looks like two possibilities in addition to those already mentioned.

Did you let your individual tiers rest, preferably with weights on them, for several hours or overnight before applying the fondant? The weight of the fondant may be squashing the cake, causing the rippling and buckling of the fondant.

Second, you say you rolled the fondant thicker. The weight of the fondant coupled with the fact that it wasn't sticking to the cake properly may have caused the problem.

Otherwise, the cake adorable!

Yes, I put each individual tier in the fridge overnight before covering with fondant. I didn't put a weight on them though - what do you use as a weight?
I rolled the fondant to 1/4 inch, but I don't think I rubbed it on enough.

I like to let my cakes settle on the counter, with a small bit of weight on tip (i.e. thick cookie sheet). A cold cake won't really settle because it is not as soft as a room temp cake. I do cover them cold though. I will fill and let settle on the counter for a few hours, then refrigerate overnight. Ice with buttercream or ganache, chill that to get the icing firm, then cover with fondant.

I need to clarify -- I do not leave my cakes in the refrigerator overnight. I apply my icing or spackle to a room temp cake then place the cake in the freezer for about 40 minutes before applying the fondant. This way, only the outside of the cake firms up while the center stays warm. There's far less condensation and what little there is evaporates a lot quicker.

Like FullHouse, I weight my cakes at room temp. Whenever possible, I'll start with the cake pan that matches the size of the tier. For example, if it's a 6-inch round cake, I'll put a 6-inch round pan over the cake, upside down - like a cap. This helps ensure the weight stays centered. For small cakes, I'll use a couple of cake pans and maybe a can of beans, or small ceramic tile. For larger cakes, I'll use a large ceramic tile, several baking sheets, or a baking stone.

I think 1/4 inch is too thick. Especially when using Satin Ice. I still think the weight of the fondant itself was a factor.

I agree with those who suggest using a fondant smoother. Not only does it seal the cake better, it yields a smoother finish. Your hand is not flat and smooth, therefore, your finished fondant won't be either.

Thank you all so much. All tips and info is duly noted. Hopefully I'll have cause to make another cake of this size soon so I can try these methods out! Thanks!

i had my only cake disaster when I tired the wooden wilton dowels. I usually use the plastic tube dowels and never have issues with them, althogh I am a beliver that sps is the best.

LOVE that cake!!!