This is my 85 300D which has been garaged it's entire life till I bought it a year ago.

As you can see the battery tray has some rust. Is this typical? It is in much better shape than my 83, where the battery tray rust is much more extensive and the body underneath has rusted through letting in water. The 85 is still solid.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Sanded the rust, washed with Marine Clean and treating with Metal Prep

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

This is typical. At least I have found so. All of my 126's have had tray rust.

In most cases I would clean the tray, repaint it with some anti-rust type prevention chemical (POR15, Eastwood's Rust Encapsulator) and then paint it with chassis paint.

I would wirewheel, sand, and treat then paint the area under the tray as well.

http://www.peachparts.com/shopforum/body-repair-restoration/288453-300sd-rust-restoration-project.html

http://www.peachparts.com/shopforum/body-repair-restoration/304380-300sd-rust-restoration-project-2-a.html

You might also want to check in the wheel well and see how the sway bar area looks. On one of the 126's it was exposed enough to allow water to splash up and into that area. I remedied this with some seam sealer.

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Painted with Gray POR-15, which very closely match the car's color!

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

My 85 looked similar. I even had a big white growth on the firewall behind the battery, maybe zinc sulfide or oxide from the galvanized sheet metal? I wire-brushed and painted w/ Rust Destroyer and topcoat. I put a Battery Mat under it, which I now use on all my cars. You can buy on ebay. It neutralizes the spills that always occur.

Your lucky, you still have s battery tray :p

Mine is mostly there but under it is a nice softball size hole....but the under coating is still in place...

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I recently had my battery tray out. It didn't have much rust and there was none on firewall. But that is probably because I painted tray and firewall with POR-15 in early 90s. Only needed touching up.

Most new batteries have a vent. On my E320, the battery is under the back seat near a lot of electronics. The battery vent tube goes through a grommet on the floorpan. Thought passed my mind that on 300D, I should run a vent tube down through the engine compartment to under car. But there doesn't seem to be much of a problem so haven't done it.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

The three W123's I have owned have all had at lease some minor surface rust on the tray or a hole in the firewall as have all of the cars I have seen in the junk yards.

Your repair should last a very long time.