[77newell]

I had somewhat similar problems on the 77 coach I owned. The really nice thing about this era or vintage is that they come apart down to the steel structure very easily. You can cut out and replace the structural steel under the water bay if it is rusted too thin. More likely you just need to clean the rust up a bit and then use a rust converting primer. The big thing in my mind is to then build up the floor in a way that controls how any water leakage flows so that it drains out at a specific point clear of the insulation. I put blocks of blue foam under the floor between the structural steel. That is not as good for insulation purposes as covering the structural steel too but I never had a freeze problem even in zero degree weather. BTW, for the floor I used plywood with several coats of WEST epoxy on all sides to basically create a waterproof tray so that leakage was contained and could not sneak around the edges and go who knows where.

One other thing I did was paint the structure with used engine oil as a rust proofing, the oil soaks into any rust still present, is cheap, and stopped rust in its tracks. In the summer if you put a coat on, then drive down a dusty road, and then put on a second coat of oil it's double protection.

If you have questions please feel free to ask, there is lots of knowledge and good will here.

[dkarb]

Wow.  You did a beautiful job!  I'll have lots of questions for you when it comes time to redo the bay.  My tanks are plastic (polycarbonate?)  My first concern was safety so we could at least take short trips for now and not worry about the possibility of a holding tank falling out on the highway.  We've only worked from underneath so far.  We replaced some of the rusted sheet metal on the bottom of the floor with galvanized sheet metal and bolted on a few I-bars to replace the original rusted through 1.5" square frame cross pieces.  This is all temporary until I can go back and do it properly like you have done.  The main 3" U shaped frame pieces are still in pretty good shape.  The rust on mine also involves some of the bay to the front of the wet bay, the stairs and the engine battery compartment.
       

[dkarb]

Thanks also 77Newell - those are a lot of great ideas when it comes time to redo the bays.  I did wire brush the remaining rust and painted it with rust converter on the areas of less damage prior to bolting the new supports on.  Oil is probably even cheaper!

[HoosierDaddy]

I use oil as well but have found Fluid Film to be very effective as well. I have bought from Kellsport in the past...Great people!!
http://www.kellsportproducts.com/

[Retro]

I use por15. Its expensive and leaves a real nice durable finish. I have rebuilt 3 coach plumbing bays and used it in all 3. I never put wood back in, it's a moisture trap. 
 https://www.por15.com/mobile/home.asp

[dkarb]

So far I've used Rust Converter by 3X Chemistry.  Has anyone used that?
http://3xchemistry.com/rustconverter2.php

[HoosierDaddy]

To be clear. I use por15 then coat with oil or fluid film. I like the hard finish por15 leaves. The fluid film has "healing" properties in the event of a stone chip etc.

[HoosierDaddy]

Dave. I've never used 3x. It looks similar to a rust converter that Eastwood sells. I have used the Eastwood product then coat with por 15 then oil or fluid film

[dkarb]

Dave. I've never used 3x. It looks similar to a rust converter that Eastwood sells. I have used the Eastwood product then coat with por 15 then oil or fluid film

Thanks.  That sounds good.  I'll do everything I can to keep this from happening again once I'm done putting all that work into it.

[whatsnewell]

Pictures of the paint job in progress on this 1981 came up on a Facebook group I belong to.....thought you might be interested.