A common misconception is that NEMA 3R is "water tight", it is not. It is not "tight" against anything but falling or wind blown rain under certain conditions. The spec for NEMA 3 is not that water will not enter the enclosure, but rather that the LIVE internal components will be protected from DIRECT contact with water. In other words, water (and thereby dust, bugs etc.) can enter, but will not build up inside to the point of shorting anything out. So if you look, you will always find a "weep hole" in a NEMA 3 enclosure (sometimes it's JUST a bottom corner folded to leave a small gap). NEMA 3R is the same, but adds the "Raintight" aspect, meaning driven rain will not get in and if the door is opened in falling rain, there is a drip hood to prevent rain from contacting the guts (but not wind blown rain by the way).
NEMA 3R also includes a salt spray test above and beyond NEMA 1, intended to simulate (IIRC) 10 years of normal exposure. So in most cases a painted enclosure will be over phosphatized or electro-galvanized steel. That just means if and when the paint eventually leaks, the steel won't rust through for a reasonable amount of time (but not forever). Hinges and latches (if any) must also survive so are usually SS or galvanized, whereas NEMA 1 would be cheapest available.
If you want something that won't rust, buy SS, plastic or fiberglass. If you want something where water, dust, bugs etc. will not enter, buy NEMA 4. If you want both, buy NEMA 4X.