G'day guys an girls, I'm alex from south Australia I've owned and been building a Mazda rx4 hardtop for over 10 years. This is my 3rd shell, she's a bit rough but has by far the least amount of rust of the shells I've owned.

I'm having some probes with rust in the water run off channel on the side of the roof an the front air vent under the windscreen. I was hoping some one would be able to point me in the right direction as to what to do about it. As it's really starting to make things difficult.

I'll post some pics so u can c what the probs are.

Cheers guys

Welcome!

Is there any reason why you would not cut that rust out and weld in new metal?

i agree, its easier said than done tho. its a situation of either trying to get hold of a roof skin, or cutting and welding but either way i dont know how to go about replacing a skin or cutting and welding such an awkward part of the body, whilst reducing the amount of filler to a minimum later

That's true...but any sort of filler in those gaps is just putting off the inevitable. Plus those edges are an important part of the structure for the rigidity of the car; you really need metal there, anything else is going to flex too much.

No chance of finding a good donor roof at a salvage yard?

Ideally id like to replace the roof, but trying to find a donor car or parts from a wrecker is difficult in australia, south australia in particular.

I do have a roof of my previous ride but its covered in surface rust and a fair bit of pitting.

Have you heard of a product called por 15?

Type por 15 in the search box. You'll find over 500 posts for por 15. Here's one thread:

viewtopic.php?f=5&t=16285&p=121538&hilit=por+15#p121538

i see two ways of cutting that out and making patch panels. one is making two pieces one for the hole run of the top and one running the whole run of the bottom. when fitted will make an L shape, and weld it in three places. the other option is to take one piece of steel, and make it as long at the entire run of the damage, and bend it into a u shape and then weld the top, and the bottom. Will look a little different, but the u shape should be even a stronger set up and minor filler needed after. dont' make it too complicated. i see two solid upper and lower edges on which to weld too. and underneath that old rot, clean it out as best as you can and yes por that thing. a quart will be plenty. then weld in your patch. after that, use some long hair fiberglass filler, to rough it it.. sand it down with 80 grit, feather out the edges etc. "this will give you added strength as well, and seal up the patch. and then use standard filler to smooth out the whole patch. will look factory or slightly custom when done.

see crude drawing attached.

alexpurcell82 wrote:Ideally id like to replace the roof, but trying to find a donor car or parts from a wrecker is difficult in australia, south australia in particular.

I do have a roof of my previous ride but its covered in surface rust and a fair bit of pitting.

Have you heard of a product called por 15?

don't be afraid of the surface rust and pitting, as compared to trying to creat and weld in a new panel this will be the lesser of 2 evils, cut the patch from the donor car much larger that you need to replace atleast 2 " in all directions, then get the piece to sit on top of the area to be removed as much as possible, (central area can be removed before hand but keep the outer edge)

now take a 1/16" (2MM) cut off wheel and cut thru both pieces at the same time about 1" in from the outside edge, your old rusty piece is not removed, and your new donor panel is the perfect side to weld in, use clecos if you have access to hold it together if not, a magnet or even tape, slowly tac weld the pieces to each other moving around to disperse heat and avoid warpage.

take your TIME.. do not try and run a bead, just small nice hot tack welds all around until its complete. grind down an EXCESS welds (you do not want to completely grind the welds, otherwise why did you put them there) scuff and prep for body work. if you want you can use POR 15 on the new and old panels, HOWEVER be aware there is a conversion coat product that MUST be used before you can apply filler or primer to the panel.

a 4" wire wheel on a hand grinder is great for prepping the metal and also removing any surface rust.

as stated above. you can do the same thing, with my shown drawing. and if your gonna loose sleep over the rust underneath, buy yourself a sand spot blaster to really get into the surface rust underneath your patch. then you can use por-15, or any quailty rust kill/even a good undercoat. it's not that hard really, you can buy your sheet metal cheap and form it as shown in the drawings. NO donor car needed.

hey alex, nice to see another aussie on here, especially into rotors, i used to have a rx4 coupe many yrs ago, loved it to death, now we have a s2 rx7 getting a respray in the shed and a 1200 coupe,
i know your pain with the parts dilemma, lol, even worse here in Tassie,
is there any chance you could find a mate who's a good welder, get some copper plate, properly rust treat the rust, put the copper plate under the holes and then slowly weld away to fill the holes up with filler rod, the copper plate will hold the weld and help stop the weld from falling thru the holes while not actally welding itself to the car.
another option would be to find another piece of drip rail from another car, don't have to be from a coupe, sedan should be fine aslong as the rail has the same profile, you only need a small piece, take it to a sheetmetal place and ask them to fold up the profile in 1 long length.
then when you get it home, you should be able to stretch the top folded part with a hammer and dolly to the shape of your roof line, pinch the very top edge of the rail by hitting it between the hammer and dolly, it will take a while but can be done, and the damage done to the top will be covered by stainless trim anyways, then just get it welded in, or take the car to a sheetmetal place and ask them to fix it from start to finish!!
any pics of the coupe??
are you on A/R with a build thread, will look it up!!
krem