Halfords do a tube made by Cataloy for knifing putty, their website mentions Isopon knifing putty in ‘Normal and ‘plastic bumper’ sorts. Presumably the plastic bumper one is flexible?
I use the Cataloy one and it seems to be ok, barring it takes awhile to harden.
Lightweight balsa filler is good, if a bit soft so needs sealing after, but I am with Richard for the larger stuff, P38.
Ashley