Here in Wisconsin, they apply salt to the roads in the winter to melt the ice. It's a REAL problem for vehicles.
I've never lived on the shore, but have heard that there are some similarities given the salt in the air from the water. If the work is done in that environment, I'd guess you'd need to take extra precautions
at a minimum. Other local body shops can probably give you better local advice on this.
From my experience, Bondo is used to fill in minor depressions and pits on intact metal. It's usually not visible from the inside, because the inside of the body is covered by the interior. Pinzgauers are different, in that what you're looking at on the inside of the truck is the inside of the body panel. You'd see most dents, so you could see where Bondo may have been used on the outside.
I had a truck that had rust holes repaired with just Bondo. From the inside, the floor looked great, but the blue goobers underneath were dead giveaways where the Bondo had oozed out. Same with the side walls - looked great from outside, but inside you could see the painted Bondo. I saw another truck with the bottom-side body support channels all filled in and rebuilt with Bondo.
IF Bondo were only used to fill in minor pitting, etc., and it was properly prepped, It's not a big deal. If it's poorly prepped or filling in big voids, that's not good, IMHO.
Others have had bad results with Rhino lining and written about it. Here's Vince's take:
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