OK, after MUCH research I have worked out what type of plastic the panels are made of. On the back of the coloured and black panels, you'll see >PP< - this means that the panels are made of Polypropylene. I don't know about XTZ panels, but here is a code chart - look on the rear of the panel for >PP< or something similar: http://www.urethanesupply.com/identify_01.php
Painting:
To get great paint results on >PP< panels you need to prep the surface really well - this means more than sanding and (grey) priming.
As a bare minimum, thorough cleaning (wash and alcohol rub or de-wax), green scotchbrite, and a tie-coat/Adhesion Promoter (instead of primer). The best ones that repeatedly come up in discussion are Bulldog Adhesion Promoter and SEM Plastic Adhesion Promoter.
The reason PP panels are so hard to paint is that release agent is impregnated in the plastic, and it seems the best way to get good results is to flame treat the surface to be painted (after the scotchbrite stage). Scarily, this involves blow torching the surface to be painted just enough for it to sweat out the release agent and then you (immediately) clean it with very strong alcohol (90%). Here is a video of the technique: https://www.youtube.com/watch?v=GTiCWp0unjs. Make sure you do the leading edge of the back of the panel too, so that the paint can wrap around a bit.
Also, many swear that 400 grit abrasive paper and water is best (instead of scotchbrite).
I've yet to try it but at least I am confident now that I have a definitive technique.
Gluing/Repair:
3M, Loctite and Selleys (Australian) have 2 part (primer + glue) plastic instant adhesive (Cyanoacrylate - super glue). I have used the Selleys product and can vouch for the results. I repaired the right tank cowl snap lug thingie using this glue and silk cloth reinforcing - it lasted well up until I dropped the bike on it (shattered). The Loctite and Selleys products come with a felt tip priming pen, so are very easy to use. Be warned - there is zero positioning time - it sets instantly with the primer.
For plastic welding you need Polypropylene rods and a purpose built welding rig (hot air or electric), BUT good results can be obtained with a soldering iron (set at 260-270degC - although recommended PP welding temp is 300degC, so try first).