Japspec
Japspec New Reader
3/1/20 10:05 p.m.

Hi everyone. I am currently in the middle of doing some rust repairs on my Miata. I have cut back the outershell of the rocker and rear quarter where I saw some rust. I've attached some pictures. Now, for the rust that is on the frame (first picture - I believe its part of the frame?) I am going to flap disc all the rust off, use some rust converter like EvapoRust to be sure, and then apply some Rust Bullet.

My question is, what should I do with the section of the frame that I had to cut out due to it being majorly rusted (second picture)? Can I just weld in a thicker piece of metal, or is there a special process I need to go through in order to repair that section?

 

WonkoTheSane
WonkoTheSane SuperDork
3/2/20 10:10 a.m.

Basic process is the same, treat as much as you can get to with it cut out, try to match thickness for the new piece and then coat the back after welding.   I've had some good luck with the Eastwoods Internal Frame Coating for the final coating of internal bits after welding.  It's got a long nozzle that shoves inside the frame and has a 120 degree (or something like it) that sprays it out in a cone.  

Curtis73
Curtis73 MegaDork
3/2/20 10:12 a.m.

I just came here to say; if you ever get bodywork advice from me it likely means I'm drunk and think I'm saying something helpful.

Never take auto body advice from Curtis.

NOHOME
NOHOME MegaDork
3/2/20 11:05 a.m.

Trying to get an idea of where exactly you are working? I cant seem to make your pics line up with the pictures of the naked Miata chassis

 

 

Here is the structure behind the outer sill. You can see the removed outer sill that I removed in once piece  to use on a friend's rusty car.  I would use a wire wheel to get as much of the ust off as possible. I would soak a sponge in Evaporaust and shove it behind the bits you cant get to, and then POR or converted to finish the job.

 

 

As you can see, that little chunk that you cut out would not make a lot of difference to the triangulation of the rear of the outer sill. Just bridging it would put most of the strength back and certainly nobody will ever see it. You do have to ask at some point "How long do I really need this to last?  If the answer is "Forever" cheap way to to buy a different Miata. Curing rust is a fools game long-term.

 

The problem with Miata sills is that they are too well sealed. This works great until water gets in from a clogged drain. The water can never escape below the round holes shown in the above picture; hence the rust line just up to the bottom of the holes. Here is a view from the rear. I suitable hole with a scupper could be installed at the bottom of the sill to let water out. The red line shows the routing of the rainrail drain tube.

 

If you like pics of  naked Miata chassis, here is a post I did a while back when building the Molvo:

 

https://grassrootsmotorsports.com/forum/grm/anyone-need-to-know-how-a-miata-shell-goes-togethe/98237/page2/

 

and do yourself a favour, while I have drilled thousands of spotwelds with nothing more than a 5/16" drill bit, these things are the shizzle

DeadSkunk  (Warren)
DeadSkunk (Warren) PowerDork
3/2/20 11:28 a.m.

In reply to NOHOME :

Pete, all those Photobucket pics are fuzzy without a 'Bucket account. Can't see them.  And Japspec is working on the doglegs at the rear of the rockers.

NOHOME
NOHOME MegaDork
3/2/20 12:13 p.m.

In reply to DeadSkunk (Warren) :

That is interesting cause they don't show the Photobucket fuzz when I look at them? Let me try something...

 

Still fuzzy?

DeadSkunk  (Warren)
DeadSkunk (Warren) PowerDork
3/2/20 12:25 p.m.

In reply to NOHOME :

Good now.

 

Japspec
Japspec New Reader
3/2/20 3:38 p.m.
WonkoTheSane said:

Basic process is the same, treat as much as you can get to with it cut out, try to match thickness for the new piece and then coat the back after welding.   I've had some good luck with the Eastwoods Internal Frame Coating for the final coating of internal bits after welding.  It's got a long nozzle that shoves inside the frame and has a 120 degree (or something like it) that sprays it out in a cone.  

Thanks, that eastwood internal frame coating looks like a good thing to have. I was wondering how I was going to get back behind the frame. I'm gonna definitely invest in that, because if there happens to be rust in the back, I want to seal it up.

NOHOME said:

Trying to get an idea of where exactly you are working? I cant seem to make your pics line up with the pictures of the naked Miata chassis

 

 

Here is the structure behind the outer sill. You can see the removed outer sill that I removed in once piece  to use on a friend's rusty car.  I would use a wire wheel to get as much of the ust off as possible. I would soak a sponge in Evaporaust and shove it behind the bits you cant get to, and then POR or converted to finish the job.

 

 

As you can see, that little chunk that you cut out would not make a lot of difference to the triangulation of the rear of the outer sill. Just bridging it would put most of the strength back and certainly nobody will ever see it. You do have to ask at some point "How long do I really need this to last?  If the answer is "Forever" cheap way to to buy a different Miata. Curing rust is a fools game long-term.

 

The problem with Miata sills is that they are too well sealed. This works great until water gets in from a clogged drain. The water can never escape below the round holes shown in the above picture; hence the rust line just up to the bottom of the holes. Here is a view from the rear. I suitable hole with a scupper could be installed at the bottom of the sill to let water out. The red line shows the routing of the rainrail drain tube.

 

If you like pics of  naked Miata chassis, here is a post I did a while back when building the Molvo:

 

https://grassrootsmotorsports.com/forum/grm/anyone-need-to-know-how-a-miata-shell-goes-togethe/98237/page2/

 

and do yourself a favour, while I have drilled thousands of spotwelds with nothing more than a 5/16" drill bit, these things are the shizzle

 

I appreciate the writeup, thank you! The approximate area I am talking about is circled below:

 

Have you been able to find a way to help with drainage? I don't drive my Miata in the rain, snow, or salt, but I do wash it.

NOHOME
NOHOME MegaDork
3/2/20 4:00 p.m.

If you wanted to drain a sill once water got in, you would need to drill a hole about where I have made the X

This picture is looking in the end of the sill from the rear of the car. Mazda sealed the sills up tighter than a duck's butt-hole. Once water gets in, the water level will settle about where the wavy line is. The most common reason for water ingress is plugged drains from the rainrail. There is enough water in there to make for a noticeable sloshing sound when this happens.

I DID actually find a way to drain the water in my case. I welded a Volvo P1800 body-shell over that Miata chassis and made sure there were drain holes at the pinch-weld! Little too drastic ¯\_(ツ)_/¯ ?

 

Pete

 

 

 

Japspec
Japspec New Reader
3/2/20 6:54 p.m.
NOHOME said:

If you wanted to drain a sill once water got in, you would need to drill a hole about where I have made the X

This picture is looking in the end of the sill from the rear of the car. Mazda sealed the sills up tighter than a duck's butt-hole. Once water gets in, the water level will settle about where the wavy line is. The most common reason for water ingress is plugged drains from the rainrail. There is enough water in there to make for a noticeable sloshing sound when this happens.

I DID actually find a way to drain the water in my case. I welded a Volvo P1800 body-shell over that Miata chassis and made sure there were drain holes at the pinch-weld! Little too drastic ¯\_(ツ)_/¯ ?

 

Pete

 

 

 

Gotcha, thanks! I think I will just keep cleaning out my drains then. Your solution isn't half bad....its actually very impressive haha! What was the motivation behind dropping the Volvo body on there? I am actually very impressed that you welded that on there by yourself!

Japspec
Japspec New Reader
3/5/20 10:04 p.m.

Alright, so after grinding off all the rust, I've so far applied two coats of Ospho to neutralize anything that may be left over. There are some spots where it has turned black, so I know its converted some rust. What should I do next before coating with rust bullet? The bottle of Ospho says to just wipe it down with mineral spirits before painting. Should I go back over it with my flap disc and try to remove the black neutralized rust, or should I just wipe it down with some mineral spirits and apply the rust bullet? 

 

Also, would wax and grease remover work in place of mineral spirits? I've got a spray bottle with wax and grease remover.

AnthonyGS
AnthonyGS Dork
3/5/20 10:07 p.m.

This thread makes me so happy I live in TX.  My 135k 94 Miata.....

volvoclearinghouse
volvoclearinghouse PowerDork
3/6/20 6:55 a.m.

Spray can of Eastwood Never Rust, spray can of expandable foam, a carving knife, a skim coat of bondo, and a quick primer/paint will probably last about 10 years.  

devil

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