Not a coat exactly, but I use ex mil overalls, flight suits etc. Very thick cotton and will take indefinate washing without shinking. Some of the flight suits are also fireproof. Not too warm for the summer and good enough to work outside in the winter, if you get a size big enough to include a thick sweater. Two ex german army tank suits and an raf flight suit are still going strong after ten years or more, though a little faded. Cheap from Ebay, steam / country fairs etc...
Regards,
Chris