GRP front panel goose pimpling

Has anyone else noticed 'goose pimple' shape little lumps appearing on GRP front panels? To be fair, our old Bailey Discovery is now 18 years old and doesn't owe us anything, but I'd like to keep it running for a while yet.

Last autumn, before getting it ready for the winter lay-up, I noticed pimples appearing in one or two places, then today whilst giving it a pressure wash, hitch greasing,etc for our next holiday I noticed that pimples are now widespread across the whole GRP front panel from top to bottom.

I'm no expert on GRP, but I don't think GRP Reliant Robins, for example, degraded in this way, did they? Or would they have if they'd survived 18 years before rolling over and drinking themselves back sober?

If the pimples are just a cosmetic flaw then it's not really a problem - although a lumpy surface will be harder to keep clean than a smooth one. On the other hand, if the pimples are the start of an escalating breakdown of the GRP then the panel will probably become much weaker and crumble away before long.

Does anyone know what process is happening?