Problems with putty

Definitely gate your filling with some tape in the future. Tamiya tape works well for this. I also often gate my sanding with tape, as not to spoil too much detail. You can rescribe the lines you've already filled, use the backside of a #11 blade. Light passes. Personally, I kee either brass shim stock or an old measuring tape strip as a guide for straight edges. Don's suggestion of a palette knife and a toothpick are spot on.

use a good tape like Tamiya to mask the 'flats' as it were, & burnish well into the seams, etc, then use filler sparingly on the 'rim'

I ordinarily use a stainless steel palette knife, but for really small areas or near existing panel lines I put a little bit on a toothpick, work it into the seam, and repeat as often as required.

Hi Tamiya white is one of the best fillers, better to apply small amounts on something like the side of a #11 exacto blade or dentist spatula. Too large amount dries quickly and doesn't go into seams well, should remain very moist until deposited. Finger application is just too difficult to get into only the target area, messy results are what you get. You can still save the model by scribing into the filler that got into surface detail areas, when it's fully dried. Good luck with it. Patrick