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Discussion Starter #1 (Edited)
Is it possible to access the rear subframe mount points from above, without having to cut into the chassis?

The corrosion really needs to be addressed but restoring just the bottom is pointless and unacceptable.

The front is no better. I'll try to get shots of all six (I think) of the mount points soon, thank you.
 

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There's no way of doing that. I would remove your subframe and sandblast it, as well as undercoat the bottom of the floor when you have it out. Then POR 15 (or whatever thick paint you could find) the subframe so it doesn't rust again. It isn't going to be an easy job though, especially if you're on the floor of a garage.
 

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Discussion Starter #3 (Edited)
There's no way of doing that. I would remove your subframe and sandblast it, as well as undercoat the bottom of the floor when you have it out. Then POR 15 (or whatever thick paint you could find) the subframe so it doesn't rust again. It isn't going to be an easy job though, especially if you're on the floor of a garage.
I'm not opposed to POR 15, but was kind of fearing this. Maybe I could get some creeping oil up through the bolt hole, if it even breaches all the way through. Do you know if POR 15 is difficult to grind off, and how toxic the particles would be (I'd use a mask, but how good of a mask and goggles would be in question)? I don't mind POR 15-ing the subframe, but I'd much rather remove the rust from both sides of the floor and mount points and seal it with the heaviest duty paint or sealing product I could obtain for a reasonable price.

So if it isn't accessible from the top, then all these mount points are destined to rust and eventually weaken to failure? I mean, it could be cut out and replaced, but then the geometry of the rear end will be altered unless the new piece is welded back perfectly. But then brings the question of how far does the corrosion go, and how much has to be cut away. This seems really overwhelming and defeating, but I am will to cut into the "frame" to access where I need to as opposed to letting it rot. Otherwise I see no point in building off of this platform, I need something solid because it will eventually be going to the track.
 

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Honestly the rust isn't that bad, sure it's ugly but there doesn't appear to be any holes from what I can see. So it's not like the subframe is going to fall out of the car.

If you're willing to spend the time to take the subframe out and sandblast it then you can do your bushings at the same time. The IS300 has pretty soft bushings in the subframe and diff, read the other thread that Ek9B18 made just recently. Then I would take an angle grinder with a wire wheel to the floor if it looks as bad as the picture does in your first post. Then POR15 it or whatever you want to coat it with.

You're really over thinking this, just get a buddy to come over and you should be able to get this done in a day easy (faster if he doesn't drink all your beer first).
 
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