Paint problem

[voyageur]

I finally had some free time and decided to clean and oil my Old Town Trapper. I pulled it of the rack and placed it on the horses. I procedded to oil the wood. Later I turned it over to oil the underside of the outwales. When I turned it over I noticed what I though was a white stain or mold. When I looked closer I noticed the outer layer of paint was blistering and peeling. I washed the canoe down and noticed more peeling, all bolow the water line. The only times I have had the canoe out this year was in the late spring and twice this summer for short trips on a lake in Michigan. I remembered that the last time I was out on the lake it had been sprayed for weed control a week before. The paint has been on the canoe for about four years without any problems. Both the under coat of paint and the outer are marine enamal from Kirby. So far the under coat looks okay. Does anyone have any ideas on what could be the problem? I have canoed this lake over a number of years, year round without this ever happening before. The canoe is stored on a rack and covered with a blue plastic tarp year round.