Hi John - watching with interest as I will have to do this at some point on my 1/18 P51C
The aircraft I am modelling is has just been restored and Aircorps Aviation posted some interesting pics I hope you don't mind me adding..
I guess they left the factory super smooth but maybe over time the filler degraded where panels but up against each other and maybe flexed in flight and that fuelled the debate about whether lines are visible or not. I don't know, and when I do mine I will likely fill like in the images, I just wish I had a pair of wings to graft on
Interesting experiment & looks like it's a winner