My experience may or may not apply here… In automotive paint refinishing back 15+ years ago, 2 part epoxy primers are special. Most primers are (were) 2k urethane. These are similar to automotive 2k clearcoat in how they work. They both have similar thicknesses, leveling after wet coats, and to a lesser extent - drying properties. With drying properties, the surface levels within a minute or so but it forms a surface film and the back side remains tacky for longer. (Where they differ is that clearcoat takes much longer to fully cure, like weeks to months, while primer is workable within an hour or less.) Epoxy primers are high build fillers. They get hard as a rock and are a pain in the ass to sand down. The two main reasons for using an epoxy are for super rough large surfaces, and this the the relevant bit here, they are used to seal the surface.

In paint, there are a ton of nightmare situations. Like let’s say some brake fluid got on the paint in a crash, or some idiot used rattle can enamel on a car. Often what happens is that the repair I am doing is not the first time the panel has been repaired. While I would like to clean the issue completely and use typical 2k primers, the previous repair may have used epoxy and buried something terrible. I’m not going to strip the whole panel and have to spray additional adjacent panels to color match when I did not estimate this in the cost quote. Epoxy seals out EVERYTHING. With stuff like spray can enamel, the painted surface never cures. It remains reactive to the solvent of anything sprayed on top of it. If ABS trim or bumper covers are exposed to acetone, similar issues with reactions will happen, unless 2 part epoxy primer is used. Epoxy primer is impermeable in the context of automotive paint; it is the nuclear option. Everything else allows some solvents to pass through it over time.

If you have ever touched the paint of new cars and noticed the softness, that is uncured clearcoat that is still venting solvents in small quantities. This is also why jams painting inside of the seals is kept to a minimum clearcoat thickness. The thicker the clear, the longer it will take to fully cure. As an ex pro painter, that softness tells me a lot about a finish too. It actually starts forming around 5 minutes after the clear is shot, just after the fingerprint test does not pull a string when removed. That is the first moment when I am able to barely graze the surface with fingertips and not damage the surface. It is still very wet underneath at this stage.

Hopefully that illustrates how even the hardest of painted surfaces is still able to allow stuff to pass through it. If you want to stop that stuff, you need a paint that is made specifically to seal everything.

That said, the seals and other materials also need to be up for the task. Most of those are likely just dust seals. How you deal with corners is critical. Just look at stuff like Pelican cases.