Sounds great Keith, and very plausible too. Looking forward to seeing those atractive Irish roundels on a P40, I agree, should look cool!
Daniel, yep, that's right. Mix Klear (or gloss enamel varnish for a thicker mix) with talcum powder, until it's like porridge. The more talc added, obviously the thicker it will be, but the idea is to have it as a thin porridge, so that it's just thick enough to be 'stiff', but thin enough to be able to run it into gaps. Hence using this instead of tube putty, or stiffer compounds such as Milliput.
With Klear, it dries quicker, and will leave a dusty deposit, which goes when sanded. Fill the gap and let it dry for at least half an hour. Don't put too much in at first, just enough so that it flows in and looks level. When its's dry, if you think it needs more, then do so, and let it build up if neccessary. Again, let it dry, then sand as normal, but lightly. If there are any pin holes or gaps or depressions, just 'paint in' some more, slightly thinned with a bit more Klear.
For this final exercise, dealing with very small depressions, that's where gloss enasmel varnish is better, as it's thicker and bonds better. Paint and talc can also be used for this final bit, although paint tends to be a bit more difficult to judge when mixing with the talc.