The inner fenders (behind the wheels) and forward frame are famous rust areas. Since its mostly hidden I have had good luck with fiberglass patch material and then truck bed liner spray to protect it. Its important to clear away whatever rust you find or it keeps growing.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma