When smoothing a firewall is it better to cut out the areas you don't want and weld in a flat panel, or is it just as good too well the panel over the existing wall?

 

Considering the complex shape of the firewall, it's probably easier to just put a false panel to cover the factory one.

 

True. By smooth I really mean just the passenger side. If I weld the panel onto the old firewall, is there any chance the area in between can rust out? I plan on anything that eats, plus wheel wells, then light bondo seams, and paint.

Just want to know what is best for the panel. I can cut out the old one, just would be nice to know that if I were to leave it in there would be no issues

 

If there aren't any signs of rust starting, I would just weld the panel in over top the original. If it hasn't rusted by now, odds are it won't start to.

 

There is zero rust on the firewall. If welding over is not a bad idea, this will save me a lot off time and effort. I guess I was just nervous about the pocket that is created in between the two layers since it won't be able to breath. But I see your point 92. Not a very rust probe spot. Plus the car will be a weekend toy, kept in a garage year round.

I will post some pics tomorrow.

 

Is the car gutted? There's insulation behind the firewall that will burn if welded.

 

Yes. 100% gutted. I have not had that insulation for years. Fat Mat and Dynamat instead. I have to rip that stuff out to weld.

 

Are you just trying to cover the holes of the heater and the ac? If so i recommend grinding the larger circle flush and then cutting panels out to butt weld them in place. There is the big circle and the rectangle which is easier and a few small circles. There shouldn't be a void in the firewall that way. it may take a little more time but it will save you future grief and look 100x better. Im in the process of doing this and more to my project car just recently ripped the entire front end and dash off and out to see what im working with .

 

I have already done that way back in the day. It didn't come out so great. It was back when I was 19. Instead of try and fix the work I have done I want to either cover it or cut it out. If covering has no negative side effects, it is a much better option for me.

 

Common practice in the custom car industry is doing like you describe with a cover welded over top. weld thru primer on the backside of the panel and anywhere you make your plug welds.

 

It will burn/smother overnight, ask me how I know.

 

Anyone know a good place to buy weld thru primer? I think this will be what I do.

 

If you're concerned about it rusting make sure that whatever you do doesn't leave pockets where water can sit. That sometimes means leaving an area open at the bottom instead of welding it shut. Even if water can't get inside, condensation can be a factor too.

 

I am definetly concerned about rust. I think anyone doing custom body work that doesn't think about rust as a #1 priority will end up with issues down the road.

I think that I will cut out some of the firewall as well as leave a lot there. Then weld the pannel onto the old firewall. That way I can paint inbetween the two pannels a bit from the inside of the car. I will buy some weld through primer as well. Leaving a large lip around the perimeter will make welding in the new pannel much easier as well. It also makes the hole I have to cut smaller. I will then use body filler to feather out the edges to hide the new pannel an intergrate it into the overall firewall. I will be skinning over the entire firewall and block sanding smooth to try and make it look as uniform as possible. I am also going to do this with the wheel wells and battery tray areas.

After that I will be painting my entire engine bay artic white.

Oh I will also be removing with lower radiator support and welding in a piece of steel. The stock one doesn't fit my circle track radiator very well.

I will post pics once I realy dive in. Right now it is just gutted.

 

Finally going to start my build back up. Just posting to remind me to post pics up here this weekend when I finally dive in

 

I would avoid welding one panel over another, it is impossible to coat the welds inside, and no matter what moisture will be trapped, on continue to sweat in there. There's a reason they don't do this for dents.

 

If you have to do this mod, vent it on the inside of the cowl where it isn't seen plus use cavity wax if it will be street driven.

 

what is cavity wax?

 

http://catalogue.3m.eu/en_gb/gb-aad/...-_Amber~nocode

Designed to give a protective coating to the inside of vehicle body panels. The product will seep, by capillary action, into gaps and along welded joints to give long lasting rust proofing.

• Remains flexible when dry.
• Water repellent - can be applied to damp surfaces.
• Excellent rust proofing against road salt

 

It seems to be used more in the UK but it's starting to become more popular here. You spray it in and it seals the metal/fills gaps so that there is no rust ever.

 

I think if you are going to take the time to do your firewall, obviously you are wanting to do it right. We have all gone the route to save 30 minutes and regretted it... Just cut it out and weld a patch in, smooth and enjoy knowing you didn't shortcut!

 

Looks like I am cutting it out. One problem area is by the cowl drain. If you cut into the firewall you end up on the drain, not the interior. Ask me how I know... this area will just get smoothed with bondo I guess. I am keeping the drain for the cowl, even though I don't ever plan to drive in the rain. Seems like a good idea. In MA you need to have wipers to be street legal too. So might as well leave the drain. I will cut the rest and weld in a patch. I should have time this weekend to really make progress. I will post before and after pics of the metal work.

 

I just welded a flat piece overy the pass side firewall, it turned out great! Then I used oem seam sealer over the welds after priming the bare welds and metal. I also made sure to prime the back side of the panel with flame proof primer then sprayed an anti rust sealant from the inside of the firewall to make sure moisture can't get between the panel and firewall and cause rusting. I have several pics of this process if anyone wants them just text me at 937-789-5505 and I will send the pics. I can't upload them for some reason. Anyways. I also did a flat steel patch on the drivers side firewall where the wiring harness comes through due to doing a full wire tu while I am going crazy with the car. I also welded ALL firewall, fender and subfamily holes closed as well so that everything is extremely clean and smooth. I have been taking tons of pics of the process to hopefully help others complete these same modifications. I have designed the wiring harness to only show 1 6in wire all other wires will be seen for a max of 3 inches before they disappear (atleast that's the plan). Feel free to ask any questions, and if you have a theory I will test it on my car and let you know how it works so it can save you time and trouble.

 

I ended up doing the same. I drilled some holes in the panel so that I can spray interior frame sealant in between the two layers. I painted the insides with weld thru primer before attaching it as well. Pretty much everything else you posted is on par with what I am doing as well.

I also decided to eliminate ALL the bumps from the driver well. There is an oddly shaped bump of the front side of it. Take up a good 20% of the entire well. I will post pics soon of what I have done so far.

my3rd - Hope we can help each other out during our builds, sounds like we are headed for similar goals.

 

Nice, I have never done body work or panel filling before so this is all a learning experience. If I was confident I could get the areas flat I was going to remove all wheel well bumps as well but I just filled the holes in them instead.

 

Same here, I have zero experience or formal training on any of this. I am just sort of having at it.

We are definitely in the same boat. I did another panel last night. Tacked only. I will post pics of where I am so far this weekend. I have all the pics on my phone, but posting them on here is a pain to do that way. Wish there was a better way. Maybe I should just upload to google drive then like when I am at work on my desktop....

 

Any pics to share yet?

 

I can take some tonight. I have started bondoing already.

 

I'd love to see some! Did you get your fenders done too?

 

OK, so here goes. This is a new way of uploading photos, so lets see if it works.

Driver side.


Passenger side


Heater core side of the firewall


I think I am about 80% done. Many thin layers of bondo on top of the welded plates. Once I am done with the bondo, I will move on to putty to further smooth things out before primer surfacer. Hope it looks slick when done.

On a side note, I bought my minitubs today. By the time they show up I should have this all wrapped up so I can move on to the rear fab work.

 

Not sure why the pictures are so small. In my Google photos account they are pretty big. If anyone has tips on a better way to upload so they don't shrink, let me know.

Thanks.

 

I'm not sure on how you uploaded it, but he URL that you used has a tag on the end that determines the size of the image.

 

Thanks for the tip. Worked like a charm!

Hope you guys like my first time bondo skills, this is still a work in progress, but at least the metal work is done. That took me a while.

 

Glad to help! looking good so far.

 

I'm no bodyman so I'm not here to critique at all. Looks great man. Did you end up cutting out and stitching in a panel for the HVAC, or did you put a plate over it?

 

I cut some places out to make it generally smooth. Then tacked a panel over the original firewall. I left the bottom unwelded. I also drilled some holes in the new panel so that I could weld thru the new panel to the original firewall. I then sprayed in between with a internal frame coating from eastwood. Before I welded it on I also painted the firewall and the panel with weld thru primer. Everything done with at least 2 coats.

Then what you see in the pics is the bondo work to try and smooth the transitions. I am also, as you can see, filling in the voids and transitions from all the sheetmetal panels on the towers.

Did another application of bondo last night. I think I probably have at least 3 more applications before I can say it is all smoothed out. To be honest, I am sort of caring less as I go because of how many components will have to stuffed in here when I install my motor, you will only be able to see small bits of this work.

Apart from the motor, there is FPR, vacuum canister, strut tower, Procharger + additional brackets, Digital 6+ box, BTM box, and all the stock stuff like brakes lines and all that.

I am no body man either, this is just me having fun trying something out. I am a mechanical engineer by training so everything I know how to do is from manuals, magazines and youtube videos!

 

Looks great so far! All the bondo/sand/bondo/sand/repeat does get to you after a while, lol.

One thing to think about, that I forgot, is where to put the engine to body ground strap. I filled all the holes in my frame rails, including that one and didn't realize it till input the engine back in and started hooking the wiring back up. It killed me to have to drill a hole in the freshly painted frame rail.

 

Ah good point. I will keep a hole free of paint and everything else. Not sure where the old one was bolted to. I may have already closed it up. Any pictures of where you attached yours 92?

 

No pics unfortunately. I drilled the hole in the top of the passenger side frame rail just in front of the heater box. The ground runs under the heater box to the back of the cylinder head. Since your removing the heater box and everything Is out of the way right now, I might just weld a bolt to the firewall, or just under the trans tunnel so the wire is hidden. Then you just need a nut to attach the ground strap. No need to worry about the hole rusting out or having a poor connection due to paint.

 

Good idea. I will weld a bolt to the floor, then mask the base area when painting.

Thanks for the tip!

 

Something I noticed the other day at the junkyard. I was pulling one of the front window tracks from a 92 RS, the one with the rubber triangle, and noticed there are 2 bolts on it that are made of copper. They are where the top angle bracket attaches to the guide rail. You could take one of those to make a grounding point. I'm not sure if it would make a difference over a steel bolt welded to the body, but it's an idea I had.

 

Cool idea. No junkyards by me.

I think I will just weld a bolt to the floor. Undercoat the weld and call it done.

I think I will be test fitting my engine in the next week or two to see how the new wheel well shapes clear the procharger. Finally something more interesting than sanding!