Refinishing Underbody - Where Should I Apply Undercoating?
#1
Senior Member
Thread Starter
Refinishing Underbody - Where Should I Apply Undercoating?
I repaired a lot of rust damage and replaced the frame rails on my S4. Then coated with epoxy primer and a 2K Urethane chassis paint.
Now the question is: Where, if at all should I apply rubberized undercoating?
I was thinking I would apply it where it came from the factory (which is the rear wheel wells and bottom of the floor pan and frame rails), plus the front wheel wheels, since I don't plan on running liners.
Any strong opinions or words of caution on this?
EDIT: Bonus Question:
Does anyone know what frame rail holes should be plugged and which should not?
I know some are supposed to be open for drainage and airflow.
In the pic below:
Green - had plugs when I got the car, so I reinstalled them.
Blue - Did not have plugs, but maybe that should?
Red - Should not have plugs?
Thanks!
Now the question is: Where, if at all should I apply rubberized undercoating?
I was thinking I would apply it where it came from the factory (which is the rear wheel wells and bottom of the floor pan and frame rails), plus the front wheel wheels, since I don't plan on running liners.
Any strong opinions or words of caution on this?
EDIT: Bonus Question:
Does anyone know what frame rail holes should be plugged and which should not?
I know some are supposed to be open for drainage and airflow.
In the pic below:
Green - had plugs when I got the car, so I reinstalled them.
Blue - Did not have plugs, but maybe that should?
Red - Should not have plugs?
Thanks!
Last edited by eyeoutthere; 11-20-22 at 09:11 AM. Reason: Thought of something else...
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gracer7-rx7 (11-20-22)
#2
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i think Mazda basically put undercoat where you painted it black
on a newer Mazda they put less on, basically they undercoat between the frame rail and the rocker panel, and then the wheel wells, so that is an option as well
i've been running without liners the whole time, and so far the only downside is that it throws dirt on the door hinges...
on a newer Mazda they put less on, basically they undercoat between the frame rail and the rocker panel, and then the wheel wells, so that is an option as well
i've been running without liners the whole time, and so far the only downside is that it throws dirt on the door hinges...
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j9fd3s (11-19-22)
#4
Rotary Freak
If weight isn't a concern, I'd put it everywhere. That's what I did, and it has a good impact on noise. Large flat panels like the floor benefit the most.
If weight is a concern, just the places where stone chips are likely.
If weight is a concern, just the places where stone chips are likely.
#5
Senior Member
Thread Starter
Thanks, Guys. I think I'll to the wheel wells and most of the floors for sound deadening.
I edit the post with another question about frame rail plugs, in case people know what's going on there.
I edit the post with another question about frame rail plugs, in case people know what's going on there.
#6
I'd use one of those spray snake things and coat inside it and leave em out so the water can drain.
#7
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this may help, or two can play that game, or whatever. this one is a 91, probably less undercoating than an S4
and you really didn't need to undercoat the trans tunnel, engine probably did that for you....
and you really didn't need to undercoat the trans tunnel, engine probably did that for you....
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#8
Senior Member
Thread Starter
#9
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Nice work!
Highly recommend some of this stuff under the transmission tunnel while the car is apart. Really helps with heat transfer into the cockpit and keeping your feet cool.
https://www.designengineering.com/fl...ld-ii-21-x-48/
Highly recommend some of this stuff under the transmission tunnel while the car is apart. Really helps with heat transfer into the cockpit and keeping your feet cool.
https://www.designengineering.com/fl...ld-ii-21-x-48/
#10
Full Member
You need to make sure you fill in the unibody as much as you can with a rust preventer. 3m cavity wax or the SEM equivalent. If not any rust repair done will start to rust again from the inside where the weld is still exposed and not coated. As for under coating I have always used rust oleum truck bed coating. Just brush it on. Works great. Also I use lizardskin heat/sound deadening spray on on the inside of the car.
#12
Full Member
Frame rails to jack up the car
I have a problem with my frame rails, the car was undercoated from the factory.
This year when I went to replace my drivetrain, I found that the two frame rails that I support the car with jackstands are collapsing!
The outside looks fine, but the inside of the rails has corroded to the point of no longer able to support the weight.
So, if you undercoat your car make sure that these frame rails are coated inside also.
Now I have to find a shop to replace these rails before my chassis folds up on me.
This year when I went to replace my drivetrain, I found that the two frame rails that I support the car with jackstands are collapsing!
The outside looks fine, but the inside of the rails has corroded to the point of no longer able to support the weight.
So, if you undercoat your car make sure that these frame rails are coated inside also.
Now I have to find a shop to replace these rails before my chassis folds up on me.
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