[hirofkd]

Looks like you are using Tamiya white putty, which is only good for thin application, like filling scratches and gaps. Also it doesn't dry in 1 hour. In fact, it may never dry if you applied it too thick.
What you need is either epoxy putty or polyester putty. Tamiya makes both, but you can find alternatives easily. Epoxy putty may be marketed as plumber's putty, and automotive body filler is essentially the same as polyester putty.
These kinds of putty cures by chemical reaction, so it's good for thick application. Epoxy cures in about 6 hours, while polyester putty takes only 10 minutes. Another good thing about these kinds of putty is that the shrinkage is negligible, while basic putty shrinks as it loses the solvent in it.

I'd use automotive body filler for this type of modification. It's cheap, available at any auto parts store, and it cures very quickly, and is ready for sanding in less than 10 minutes. The tricky part is that both epoxy and polyester don't adhere to plastic very well, and you'll probably have to remove the cured putty and re-attach it with super glue.

If you have to use Tamiya putty, don't apply any thicker than, 0.5 mm, then wait for overnight, then add another layer. Keep building layer by layer, and eventually you'll have enough thickness, but I really recommend epoxy or polyester putty for any type of body modification, and leave basic putty for finishing work.