In damp weather, a small quantity of water vapour is absorbed into the paint structure and is then evaporated again in dry conditions (osmosis). This process is normal and does not harm a well constructed finishing process. However, poor processing of the primers will leave hygroscopic/water soluble substances (salts) behind as contaminants. These cause a local concentration of a salt water solution which lift the paint film into water blisters. Blisters can occur in many sizes, patterns and frequency and can form between individual layers or beneath the entire film build. In dry weather most blisters will temporarily recede.

Causes:
-The surface to be painted (filler, bare metal etc) was not cleaned thoroughly. Contamination from salt residue, eg dirty sanding water or hand sweat, was left on the surface under or between coats. The blister pattern may indicate the cause (beading = wipe marks, prints = finger or hand prints).

-Wet sanding operations (particularly with polyester based products) without sufficient time to allow water to evaporate before application of top-coats.

-Dirt on the surface which acts as moisture absorbing sponges, once heat is applied or are exposed to certain temperatures, these particles expel water resulting in blisters.
-Moisture absorption by the coating, either by lack of crosslinking during the curing process or by absorption of porous substrates prior to application of paint.
-Osmosis phenomenon through the paint layer, so that it absorbs water and produces the blisters.
-Paint on very cold surfaces in comparison with the environment, so that the ambient humidity precipitates forming water particles on the surface, which generate blister when the paint is applied.
-Paint application in high humidity environments.
-Insufficient drying of the substrate or previous layer of paint which contains moisture prior to application of paint.
-Water sanding on paints or polyester putties, not leaving enough time for complete evaporation or drying the sanded layer.
Prevention:
-Thoroughly clean the areas to be painted with  clean water. Change sanding and cleaning water
regularly, especially in winter when vehicle  paintwork in for repair is covered in salt. Consider
changing to dry sand operations to avoid water  absorption when sanding. Finally (depending on  the substrate) clean the area with an appropriate  cleaner.
– Allow sufficient time for water to evaporate when wet sanding (circa 2 hours at 20ºC). Dry sand  where possible especially with polyester based  products.
– Ensure booth is dry before painting. In cold damp  conditions run the booth on bake before any  paint processing operations.
Solution:
Sand down and remove the paint layers back to a sound substrate. Be prepared to strip the areas back to bare metal where necessary. Refinish with the correct choice of primers fillers and top-coats.
REFERENCES
1.CoatingsMall, Paint Defects Advice, date of access: 21 June 2016 https://www.coatingsmall.com/cap_data_docs/documentum/Training%20(BASF_Training)/Demo/Paint_Defects.pdf
2.Painting for Painters , Blistering, date of access: 21 June 2016 http://www.paintingforpainters.com/blistering-paint.html