I finally bit the bullet and bought new fuel tanks. My driver's side rusted through and leaked at around 3/8 full. Both tanks were full of rust, continually clogging the fuel filter. I went ahead and bought a set of Spectra Premium tanks. They came galvanized but with no paint. I wiped down the tanks and sprayed them with undercoating. However, the OE tanks seem to have a layer of sound deadener on them. It looks like a sheet of mass loaded vinyl (MLV) glued to the side of the tank. Looking at the leaky tank, I'm wondering if perhaps this failed MLV caused the tanks to rust through by retaining water/moisture in one of the cracks. It's also possible that it rusted from the inside out. I'll never know.

Anyway, any thoughts on this? I have some "Dynamat" (actually Stinger Roadkill Expert) that I could cut and stick on the sides of the tanks. The MLV used on the OE tanks seems to make a pretty significant difference. When you tap the OE tank, you get a tight thud. When you tap the new Spectra tanks, you get a hollow bong. (No, not that bong!) I know the tight thud is acoustically much better than the hollow bong, but will I notice a difference when driving/using the car?

 

It's a long time since I had the tanks out on a Series 3 I had, but I think there were a couple of pieces of sound deadener on the tanks, but not very big. It's there to stop drumming when the tank is empty or getting so. Most times with these cars, one tank is always nearly empty !

 

yes, it cant hurt anything and since you have the stuff, I say do it.

 

What did you use as "undercoating"?
Make sure it's impervious to water and applied well. I would use epoxy primer.

 

Well, the galvanizing should delay rusting way beyond your needs. And, the undercoating should calm any resonating so, I'd just put them in as is!

 

Thanks for the help guys! I used regular undercoating from a spray can that is used on the underside of cars. I'm sure it added some mass, but the tanks still resonate. I think I am going to put some on and see how it goes. I'm going to try to do it in a way that will not retain moisture. I'll post back to let you know how it goes!