I'm with John regarding 'Milliput'.
I've been using it for probably forty years, and never use other 'modelling' putties, apart from home-made 'porridge' (gloss varnish and talc) for very small gaps.
John uses the 'Milliput 'white', which is really designed for repairs to porcelain, china etc, although, of course, it works just as well on models.
I prefer the standard yellow-grey type, which has a little more latitude when used in very small, thin quantities, where the 'white' might start to flake, especially when used wet. Also, the standard type seems to set quicker, and easier and, being in two colours, it's easier to see if you have the 'mix' right.
Oh, and it's cheaper than the 'white' too !
As it's a two-part epoxy ( 1 stick grey, 1 stick yellow), there's very little waste, as you can 'mix' as little or as much as you need, and a little goes a long way.
As John mentioned, it can be smoothed-out to the point of almost not needing sanding, especially if 'moulded' with a wet finger tip, and can be moulded into shapes, drilled, tapped, carved and sanded. Apart from use as a filler, it has many more uses, and small parts can be made from it, and items such as seat cushions, parachute packs etc etc are easy to make, just use it as you would modelling clay. Even conversions can be tackled, like the entire front end of the 1/32nd scale Spitfire Mk.IX below, made entirely from 'Milliput'.
Filling gaps is easy - just press the putty into the gap, smooth it off with a wet finger, or a wet blade, and wipe off any excess.
It sets rock hard, with none of the 'powdering' or 'flaking' you get from some other types of putty, but it's easy to sand and carve as required.