Here is my putty/filler applicator...

Thanks my friend @johnb!

Thanks my friend @neil-foster! Surprisingly, these are commercially available at supermarkets, at least here in Greece. This is their latest (last month) packaging...

Nevertheless, it might be possible to "fabricate" such a "spatule" from a similar thin metal foil-style piece and, of course, I will be more than happy to send you a pack of those razor blades.

I remember my dad using a razor that had these blades, you twisted the handle and the head opened to accept the blade then you closed it to shave , he always ended up with at least one bit of toilet tissue stuck to his face with blood where it cut him ! , didn't know they are still available , good tip by the way mate.

Thanks my friends @neil-foster and @bikequeen!