HELP: How can I make smooth a vase object's inside without modify the outside?

[Anonymous]

Hello,

I'm trying to make a piggybank from a sculpture that I can 3d print but as you can see on the pictures I have some trouble with the eyes and the ears in the inside. 

I would like to make the inside as smooth as possible , but if I remesh the backfaces they always change the outside too.

Can you give me some advice how can I fill the gaps and make the eyes printable?

Thank you for your prompt answer!

[lichtzeichenanlage]

I would do it in Meshmixer. This video (https://youtu.be/Rhyf_JttQEY) shows how to hollow an object and this video (https://youtu.be/NhLREmjomAY) show how to make a piggybank. (Sry for just posting bare links, but the edit icons (insert video, insert link etc.) didn't show up.)