Thanks both for your help - I don't think the problem is connected with the original re-fin as it didn't arise until recently - the bass was fine for about 30 years. There could be a connection with the re-foam - maybe the new foam is having a reaction? I'm not sure about this as the bloom occurs with the bass in a different case, Also, there doesn't appeR to be damage to the finish - the bass still has a vibrant shine when the bloom is washed away (though I'm not keen on using hot soapy water to clean a bass!) I think
I'll clean it this week then leave it out of a case to check if the bloom appears. If it's some form of component gassing, the air circulation should prevent or reduce blooming. It's a bit of an odd issue - a shame too, the finish is really lovely! Thanks again, much appreciated and further ideas welcome!