eyeoutthere

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!

j9fd3s

iTrader: (3)

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...

Spider2k

if it were me? ALL of it.

nice work btw, that **** is sexy.

WondrousBread

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.

eyeoutthere

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.

Spider2k

seems odd to have some plugs but not others in the same opening.

I'd use one of those spray snake things and coat inside it and leave em out so the water can drain.

j9fd3s

iTrader: (3)

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....

eyeoutthere

Extremely helpful, thanks!

gracer7-rx7

iTrader: (16)

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/

Rotarybud6

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.

Turbonut

iTrader: (1)

When we purchased our '88 TII and then the '89 TII new, there was no factory undercoating, the dealer
did the job. They did such a lousy job on the '88, I told them not to spray the undercoating on the '89,
I did it myself.

gsmithrx7

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.

gsmithrx7

Hi everyone, since this thread has to do with protecting the underside of the car, I have a related question.

I am installing a new driveshaft and want it to stay rust free for as long as possible. What type of rattle can paint holds up the best for this application?

Would an epoxy paint be better than engine enamel one?

I am cleaning the new u-joints of surface grease and what little paint was applied at the factory. Then priming with a high temp primer.
What I hope to accomplish is to prevent any rust from spreading to the needle bearing by sealing the gap.

Your insights are appreciated, thanks!