What do you guys do to the underside of your mustangs?

[cst 13]

DP90 followed up with Monstaliner rolled on Dark Gray

[stangs-R-me 84]

I just sprayed them down with degreaser to allow the the factory red-oxide show ... finally got around to doing this in 2011.   

As you can see from the pics, it is in really nice shape from the firewall back to the axle, there is surface rust behind the axle though.   It was an Atlanta DSO car and had VA plates on it when I got it in 1981, so quite possibly never saw much winter snow or salt.   The car was someone's daily-driver with 110k miles on it, a dull silver paint job with dents all around, smashed windshield, and no engine/trans.   Car has around 155k miles on it now.

Doug

[Vicfreg 495]

PPG Primer and Single Stage Black Satin

 

[danno 124]

The real answer to your original question depends on what the underside it like now and where you live.  I bought my 69 in 1983 in Dallas and drove it to Minneapolis.  I only drive it in months of the year without an "r" in the spelling:  May, June  July August.  I have never touched the underside, and I would swear it has not changed in driving 100,000 miles for almost 40 years.  Rust hides quite well, so it is possible I do not see it, but nothing has shown up.  

So just perhaps, in your case, doing nothing is the answer???   I'm just sayin....

[Vicfreg 495]

Doing nothing is definitely the best option. In my case my car sat in a field in Florida for years, with water collecting in the drivers floorboard that eventually caused me to replace half of the floor section.  
I envy the folks that have cars with the original patina that shows years of normal wear and tear...it’s only original once.

 

 

 

[Vicfreg 495]

 

[69Stanger408 80]

I applied Raptor truck bed liner