The left front fender on this old British car definitely has seen drier times. The lower quarter is so deteriorated with rust, that whole sections are missing and it is nearly impossible to define its overall shape. The whole section will need to cut away and replaced.
The quick and easy way to fix this would be to purchase a used fender in better shape and replace the whole unit, however the going price of a used unit on eBay is $250 to $400 and that doesn't include shipping. That is $240 to $390 more than I wanted to afford ... shipping included!
What I have done here, are the steps taken to recreate the missing areas and then form a new patch to replace the rusted area.
Thoroughly clean both sides of the rusted area with soap and water to removed any dirt, oil or grime. Carefully, wire brush away as much of the surface rust as possible. Leave as much of the metal as possible, even if it is razor thin and can be bent with a sneeze. You will need every little bit to help recreate what was the shape of the panel. The photo shows just how little metal was really left. Do not remove any paint or filler on the face of the area...that may be the only thing holding the rest together.
Next, come up with a paper template of what the shape should be. I was able to find an old fender from the opposite side of the car and with a sheet of wrapping paper and some carbon soot, was able to make a rubbing of the area. Using spray adhesive, glue the rubbing down to a piece of poster board or, as I have done here, pieces of adult beverage cartons that somehow, mysteriously appeared in my shop. Cut out the image and keep this as a master template for two reasons: a) Mistakes can be made and you will need to rely upon it again and, b) you will probably need this for the other fender.
Using the template, trace and cut two more sections of poster board.
Trim each poster board piece to fit snuggly and conforming to the curvature of the inside of the fender. Coat the inside of the fender and one side of the poster board piece with spray adhesive, and when the coating is tacky press it into place. When that is dry, repeat with the second cardboard piece and tape in place. This will help hold the curvature and strengthen.
Turn the panel over and work on the front.
Lay several layers of masking tape to give the area some uniformity and more strength.
Cut 1" strips and tape in place around the outer edges to form a mounting lip and fender inner lip.
Coat the entire area with a coat of catalysed resin to give a workable surface and let it harden.
Coat the entire area with body filler, sand and shape until you have the look that you want. Make sure to fill all small holes and sand smooth with a very fine grit sandpaper. Coat the entire area with several coats of car wax. This will be your 'plug'.
Coat the area with vaseline or any other release agent, lay on the fiberglass cloth and coat the glass with two coats of catalyzed resin.
When the resin has fully hardened, slowly and carefully pull the fiberglass away from the fender. This is the mold by which you can cast your new patch. Clean away any release agent from the mold. Fill any air pockets and voids with bondo and sand smooth. Recoat the inside of the mold with several coats of car wax and release agent, lay in another layer of fiberglass with several coats of resin. When this hardens, carefully separate this from the mold.
Trim the excess glass away, fill all the voids with bondo and sand smooth. The front or face of your cast with be an exact replica of your plug, so any additional fiberglass should be added to the back of inside of the cast.
Cut or grind away the rusted area from the fender. Remember to leave mounting faces on each piece in which to rivet and bond the new patch.
Clamp in place and mount with pop rivets or screws, mold it to the rest of the fender.
