when i first bought this car i covered the hole up with a piece of flex tape just so i wouldnt leak. its been a year or more and i finally got to filling it in. the red piece is a patch cut off a '81. was able to make patches for the main hole and rusted areas of the weather seal lip with it.
the rot comes from this weird foam stuff mazda sprayed on the core support that holds the latch mechanism. its spot welded to several panels in the hatch area. it seems they had two raised spots that didnt touch the panels but were close enough they were worried about vibration? anyway this stuff decays, gets wet, and i think had direct contact to the metal without primer. rot city. it created the main hole and serious pitting on the other side. i removed the core support for access on this job. ill be welding it back in today.
i started by filling in rotted sections of the lip to get a hang of it, since they are covered by the hatch if i messed up a bit it wouldnt matter as much. there were several spots like this. 4 or 5.
tig is perfect for this. more pen with less heat, add filler as you feel its needed. no spatter. back off on the heat as youd like. all great things on thin sheet metal.
with those in, i got to work on the main patch. i cut out all rust roughly with an angle grinder, and then cleaned up the edges with a carbide bit then belt file to make straighter edges.
using masking tape and backlight, i traced the form for the patch, and used geometry marks to transfer it onto the donor piece
then the patch was rough cut and shaped until it fit snugly, tacked, and welded. it only took a tiny bit of bending to match the curve on this section. i used magnets to hold it in place while tacking
a die grinder with roloc pads flattened everything out. i went a little heavy on the heat on the left side of the patch. that got quite warped. a hammer and dolly got most of it out, but bondo exists for a reason. ill be doing 2k primer first, then bondo, then primer again. no paint match this time. the whole car has roached paint anyway and the primer is black.
you might wonder why i didnt do a square patch. the donor piece is from an '82 and i realized after getting it that the hand groove for the hatch has a different stamping. the u shaped bend has the same lines, but the lines inside the valley do not match up. if i cut a square patch i would have had to rectify that. would probably have taken hours with a hammer. i elected for a round patch only where it was needed instead.