I have used Nitromoors all purpose paint and varnish remover..You have to be cautious when using this..Literally put it on a sponge work in and wipe off more or less straight away..Also once I am done with an area I neutalize and just put some water over the area the Nitomoors was on and dry it off.
http://www.duluxdecoratorcentre.co.uk/w ... &langId=-1
It does a great job but do not use for exposed lengths of time to dry as it may start eating into the fibreglass..It's strong stuff but I have stripped 2 helmets and the results are really good.
Also I hear others use brake fluid, I have never used that though.. My advice is with anything..test a small area on something othar then what you want to strip first, then proceed once you are happy..
Good luck..