Weid resin reaction

 

I need a spare locomotive for the NG7 layout, and in the cabinet, I have this model, built from a Smallbrook studio kit many years ago. A quick wheelclean and lubrication, and she's good to go. 

But, the tank sides both exhibit a weird bubbling under the paint. 

Does anyone have any idea why? The rest of the model is fine. If I've followed my normal practise, the resin body will have been primed with Halfords primer, then the the top coat is enamel. 

For the moment, I'm going to pretend it's a clever weathering technique.