[Lex]

You are looking at a lot of putty and a lot of sanding. Sanding works best when the object (shell) is rigid, so one can not simply build the hull as-is, and then apply additional putty to it and expect it to work - the skin will deform during the subsequent sanding and you will not be able to achieve a smooth surface no matter how hard you try. The common ways are to provide support to the skin by either (a number of) additional layers of card, resin cast, balsa, expanded foam fillers, or anything else you see fit, before using the putty. I will leave the details to experts of these construction - I am not one of them.

What I can help is on the double bottom construction, which is also worth considering. It's quite popular among some of my friends and is sometimes even included as part of the ship model itself nowadays. It essentially backs the skin with additional layers of card, it requires little sanding and filling and uses the original skin parts. For an example refer to here:
IJN Akizuki 1944 [Answer 1:200]