Fixing what you should have fixed before paint
 

So 'my friend' has a bonnet that is fitted and painted and looks almost alright.

Unfortunately the 'almost' bit is really irritating.

There are a few high and low ridges that show if you look at the right angle. If this was still in prep then I would fill and sand but now it is painted.

Does all the paint and undercoat need to come off right back to the glass before starting the filling and sanding or will filler bond onto the paint/undercoat?

I know that either way it will need repainting, I just need to know how deep to sand, I mean my mate does, I would never be so silly

Nick your mate is very fussy and the car looks great - black always shows every tiny sin

If the marks are as bad as you say then taking off the high spots will take you through paint into primer and maybe the gel coat below.
I would advise removing the paint from the low spot to make sure the filler gets a grip properly.

I am getting a new 250SWB bonnet soon so I will be able to swap between the two. My thoughts were to get the 250SWB bonnet properly prepped and painted then whip off the Z300S bonnet and 'improve' it.

I was thinking a pale mist coat and then sanding to see where the dips and dents are. Having looked at exactly how bad it is I am hoping I can get away with a high build primer so hopefully just sanding enough to abrade the paint will do. If the ripples at the front, the worst bit, do need more then I will take the dips right back and fill.

Um, I mean my 'mate' will :-)

are you or you mate :P about on Saturdy im venturing down to see Chris