I assume you are talking about the fender extension and headlight door???
These were originally made pot metal and if damaged are really hard to fix.
I have always used Klean-Strip which is an aircraft paint remover. I used this on two 65/66 Mustangs and one car had 5 coats of paint. It took a couple of applications to get to bear metal though. Make sure you use it in a very well ventilated area and use a respirator that is rated for toxic fumes and solvents...also use rubber gloves and safety glasses and don't wear good clothes or shoes...after stripping to bear metal use a metal prep to prevent rust..IMHO