We had visitors stay over the Christmas period who had a recently purchased SUP board ( I won't disclose brand ) the board had bubbles on the bottom .
our friends had bought it new and several of the bubbles were on it when they open the packing it was in , they rang the seller who flogged them off sayingt They would go away , they naively accepted this , only to discover they increased once they started using t he board .
they again contacted the seller who asked wether they kept it in a bag they bought with the board and were told that this was probably the reason .
They suggested that this was correct as stated on various forums on this sight .

I believe many of the statements I have read on this forum to be absolute nonsense and a possible cover up for faulty workmanship or use of probably cheap unsuitable pruducts .

I owned a fibreglass business building yachts for many years and during this time there were a lot of experiences in boats getting osmosis , the main cause of osomisis , is bad manufacturing practices or environment or inferior or non suitable material use .
i believe even the well know brands are made in Thailand which at certain times of the year is very humid , ifs the humidity is not taken out of air in the manufacturing area the likely hood of the board contracting osmosis would be high
, the same as it would if cheap unsuitable or inferior finishing coats are used .

IF as some of the replies on this forum suggest that it's because of the board being stored in a bag sold by the same company that sold the board , then I would suggest that the boards must be of such poor quality and being manufactured with products not suitable for the applications they are being purchased for .

I've had a few boards now, all of which have been Thai built and some I have wet sanded the bottom with 2000 grit wet and dry. Every one of the boards I've sanded has had some bubbling in the paint finish.

As a matter of course, I take boards to the water in a bag, but I never take them home in a bag wet or salty. This will only make the bubbles bigger.

A big reason the boards from the big players come out with fancy low gloss or flat finishes in multiple colours is more for the fact that they are brilliant at hiding imperfections than marketing and branding purposes. High gloss finishes in high humidity are not easy to achieve.