I've never seen a 'professional' painter who didn't wet flat the primer. Yes it is porous so the top coats can bond to it but as long as the surface is dried before top coats are applied there shouldn't be any problems.
Professional bodyshops use ovens so after wet flatting when the car or panel is in there being painted there is not much chance of trapped moisture.
I use a heat gun to warm the panel and make sure there is no moisture there before top coats are applied.
I have painted four full cars in DIY conditions, all were wet flatted and none of them had any problems with rust coming through or reaction problems. I had them for years after they were painted (one of them I had for 10 years after I painted it.)
Professional bodyshops use ovens so after wet flatting when the car or panel is in there being painted there is not much chance of trapped moisture.
I use a heat gun to warm the panel and make sure there is no moisture there before top coats are applied.
I have painted four full cars in DIY conditions, all were wet flatted and none of them had any problems with rust coming through or reaction problems. I had them for years after they were painted (one of them I had for 10 years after I painted it.)
