If you are using a 2 part paint system, if the catalyst is mixed correctly then the only thing that can soften it is a paint remover. acetone or thinner once cured shouldnt have any effect. 

However even this, I believe, is no longer really relevant. Old style cellulose primers are likely the culprit, as cellulose is naturally hygroscopic. Hence spray painters have been coached into treating primers as non-sealed substrates, which when you are painting a car for $15,000 does make sense, you follow certain procedures. Polyester primers are a similar story, generally the rule is you ALWAYS dry sand polyester coatings and allow them to cure overnight before commencing wet sanding otherwise they absorb water. That isn't just primers though, it's any polyester coating, and it isn't due to porosity. 

                Very well put, your advice is always on the ball. You are an asset to this forum.....