At the risk of sounding like a complete moron (unfortunately not far from the truth), I wonder whether rusted frame rails can be replaced or repaired with "new" welded in pieces of steel? I'm no welder or metallurgist, but it just seems that grinding off the rust and coating it with POR 15 or something similar is kind of temporary and will have to be repeated annually. I'd like to figure out something I could have a welder or a body shop do that's permanent.

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I never go far without a little Big Star

1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k

POR-15, when properly applied, should be virtually permanent. The only thing I'd be worried about is that the structural integrity is compromised and it needs replacing, but I don't know how one would measure that.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition

Any frame pieces can be obtained from Mercedes, as far as I know. We routinely replace frame sections with either factory sections or fabricate our own. This is something that either a body shop or fabrication shop should be able to do with no issues. Now, how much it will cost may get scarey.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K