GRP sides a bit tight (in places!)

[jps]

I've been dry fitting my GRP sides and found that they vary between being just right, too loose and too tight.

This is probably partly because I have gone for a 3mm alu floor and there are some areas where I can dress a couple of welds and maybe get 1mm back to work with...

Just right I can deal with.
Too loose is not so bad either - thanks to seeing how Sylvain has dealt with similiar challenges here: http://forums.haynes.co.uk/showthread.php?t=13363

The 'too tight' bit has already led to me putting a small crack in the gelcoat - presumably from trying to flex it too far - so I am not going to force the panels in place.

Any tips for what options I have to make things fit? Can I sand out the inside (i.e. the non-gelcoat side) of the panel to introduce a bit more room?