So far I've filled the suspension arms (M3 left the factory with solid arms rather than the pocketed ones on the M4):
They were filled with P38 car body filler, then topped off with Upol Glaze to get them absolutely smooth. I still need to re-profile the ends, flatten the underside of the spring pockets and add the bolt head for the slider pad.
Next was the final drive casing. I must admit I uttered a few choice words when I saw it had been chamfered instead of filleted, but it's very easy to build up the correct curved edge using more car body filler.
I used a wood rasp to take off the high points, then a wet bath and 120 grit wet&dry paper to work the putty back to the required profile. I still need to sand back the machining marks and add the grub screw drain plugs in each side.
Next I moved onto the bogies. I've created the correct rounded profile between the top and the front face, using a cheap electric file to take the bulk of the material off, then hand files and thin strips of emery cloth to round off and blend in the profile.
The tops of the webs still need to be rounded to the correct radius, plus the centre two need the step building up a curved profile. I'm still wondering whether to create the two cut outs in the upper webs

Adrian.