If you want to neutralize Oxalic acid then use Borax (dissolved in water).
Having said that most people don't bother and you certainly don't need to use it when you are doing a job on GRP; lots of freshwater will get rid of the Oxalic acid.
I've only ever followed up with Borax when I've been varnishing wood. It's worthwhile to be absolutely sure that all the Oxalic acid is removed from the wood.