floorboard rust -what is that coating?

[hls3073z]

I'm digging further and finding that I will need to replace the floor pans or at least most of it.  It looks like the rust problem was caused by the fact that water got up from the bottom and then underneath that rubbery coating.  That coating kinda looks like a heavy duty roofing membrane.  You can see pieces of it ripped up in the photo.  I suppose that this stuff was put in at the factory since its painted 918.  Was this put in to be a noise dampener/insulation?  Odd that there are drain holes that actually served as a way for water to get up under that coating.

[SoCalJim]

Yes, it’s a sound deadening mat.  The drain holes should’ve been plugged with rubber grommets.  

[Patcon]

It is an asphalt based sound mat. Sometimes it will just chip out. Others have used a heat gun and chip it out, but some have had really good results with dry ice, then chipping it out.

[SoCalJim]

I just chipped it out on my Z.  Retired, lots of free time on my hands.  ?

[hls3073z]

I'm retired too so I'm slowly finding all these problems.?  Just for reference, there were drain plugs in there, they were just old and brittle so I pried them out.  Unfortunately, water got around those plugs or just came from somewhere to cause that devil rust in the floor pan.  Disappointing since its all my fault since I've had this car since  ' 76.

[Patcon]

1 hour ago, hls3073z said:

I'm retired too so I'm slowly finding all these problems.?  Just for reference, there were drain plugs in there, they were just old and brittle so I pried them out.  Unfortunately, water got around those plugs or just came from somewhere to cause that devil rust in the floor pan.  Disappointing since its all my fault since I've had this car since  ' 76.

Many times the water is from bad door seals, cowl drain tubes cracked or mis-installed or rust in the cowl area. Especially around the fresh air vent stack. Normally rust from the plug holes themselves eats the holes larger and the plugs fall out

[hls3073z]

11 hours ago, Patcon said:

Many times the water is from bad door seals, cowl drain tubes cracked or mis-installed or rust in the cowl area. Especially around the fresh air vent stack. Normally rust from the plug holes themselves eats the holes larger and the plugs fall out

Are the cowl drain tubes plastic?  I can spray some water down that front cowl and then see if any comes out inside.  I think the other thing that happened is that there was some aftermarket coating on the bottom that had developed cracks so water got trapped underneath leading to rust from the bottom.  However, its clear that some water came in at the front part as that is where its most rusted out including that rail.

[Patcon]

1 hour ago, hls3073z said:

Are the cowl drain tubes plastic?  I can spray some water down that front cowl and then see if any comes out inside.  I think the other thing that happened is that there was some aftermarket coating on the bottom that had developed cracks so water got trapped underneath leading to rust from the bottom.  However, its clear that some water came in at the front part as that is where its most rusted out including that rail.

They are a thin rubber tube, like a radiator hose but thinner and no cording. They can dry rot and crack or come loose from the drain neck

[87mj]

I realize you didn't photograph the entire pan but from the photos, it looked like surface rust that as you said earlier, was cause by water getting between the sheet metal and the tar mat.  Are you sure you pans need replaced? From your photo, they don't look that bad.  Maybe blasted and painted only?

[cbuczesk]

17 hours ago, 87mj said:

I realize you didn't photograph the entire pan but from the photos, it looked like surface rust that as you said earlier, was cause by water getting between the sheet metal and the tar mat.  Are you sure you pans need replaced? From your photo, they don't look that bad.  Maybe blasted and painted only?

I agree. They look really good. Try wire brushing and sanding them.

Chuck