The bad news, if I'm right, is that the metal is rusting from the inside out and the entire panel/section is rusting. The three small crusty looking spots up and to the right in the picture look to be rusting spot welds under the paint. Scrape a knife across the crusty spots a bit and odds are your going to break through the paint to rust.

The 'putty' might be smoothing putty applied before painting at the factory. Or it might be a quick and dirty repair to cover up the rust. I'd think that if you bought the truck new it is the former, if not, the later. Doesn't make much difference.

I suspect that no matter the history if you want a 'cure' the panel will have to be replaced. Big$. Given your location and the likelihood that the roads get salted I'd give you odds that there are similar rusted spot all over the truck. It wouldn't make sense to spent a lot of time and money patching one panel only to find out you have similar, or worse, rust in a half-dozen other spots. I'd go over the truck with a fine-tooth comb looking for signs of rust before committing to an extensive repair.

The bad news is that once you start digging odds are you are going to find more rust than makes sense to repair. You still might dose it with Os-Pho, a phosphate based rust converter, or similar and slow it down a bit, get a few more years out of her but it is usually a rear guard on a lost battle.

http://www.ospho.com/

We used to hose the interior of rusting panels with one of the many home brews. Typical ingredients included wax from toilet rings, diesel fuel, roofing tar, and kerosene. Anything that anyone had on hand and though might slow down the rust. Modern rust converters are better but you have to use them first if you go that route.

Vehicles don't last long if they get salted and it gets up into the body panels. Rusting from the inside out is insidious, by the time the rust shows the damage is done.

I've seen similar things with vehicles driven on the beach down here. The salt-water does a number on them. Even worse down here because, like most chemical processes, rust works faster at higher temperatures. They look fine then a year after the first rust spot show up the vehicle is blown through with rust.

The good news is that its a truck and rust, primer and rough body work, look better on a truck than a car. Being a truck it likely also has a structural frame which the body just rides on. So you can pretty much run it until it fails to cast a shadow.