Rust Proofing Trunk Undercarriage During Replacement of Gas Tank

[DylanFan52]

I am in the process of a beginning restoration of a 1959 Series 62 Cadillac.  One of the first things I am doing is installing a new gas tank and fuel lines.  After removing the gas tank I noticed some surface rust on the underside of the trunk, the space where the installed gas tank meets the trunk underside.  Solid metal just some surface rust.  I would like to do something to prevent future rust before installing the new tank.  I have thought about Rustoleum primer and then undercoat the space after scrapping the surface rust with a wire brush but wondered if I should try to eliminate the rust before covering it.  Any thoughts or suggestions appreciated.  Thanks in advance.

[Lucas Feininger #15674]

I think your idea of lightly sanding/wire-brushing the rust and then painting with a couple of coats of rust inhibitor (Rustoleum, Eastwood Rust Converter, POR-15, take your pick) ought to be just fine. Don't go crazy with the rust removal, as these products generally combine with the oxidized steel to halt the rust.

That area doesn't usually get that much exposure to the elements to begin with as the gas tank shields it and does not need to be overly protected.

[Bobby B]

If you're not going to go crazy with it, just loosen all the rust scale, wipe it down, let it dry, and go for it. I'm not endorsing any specific products here because it's a matter of choice, but if you just want some protection and you're going to brush it on, Rustoleum Rusty Metal Primer in the can will do the trick. Just remember that it is oil-based and takes a little bit of time to really dry depending on conditions. I would top-coat it for a little extra protection before putting the tank back up. I used to be a big fan of the POR-15 stuff until I had to remove it, which is next to impossible. The fumes are nasty and if you get any on you, good luck wearing it off. The underlying surface has to be prepped properly or it peels off in sheets. Once you open the can, it needs to be closed immediately and don't get a drop in the groove of the lid or else you won't get the lid off, and you've thrown away a lot of money. I have cans of it in my shop and have stopped using it for all the above reasons. Rustoleum will work fine if you just want some protection as stated.  Good luck!
                                                                                            Bob

[Scot Minesinger]

Bobby or Lucas's posts will be just fine.  I replaced my rear shock bolts 9 years ago on my 1970 Cadillac with std cad bolts and have driven 34k miles, they look new.  Point is your car is unlikely to be driven in wet weather very often and once you proceed as described this is a lifetime repair, given the use anticipated.  I have done just as Lucas suggested and although I have not checked other places on my chassis that are more exposed still look perfect.

[55 cadi]

Hello Dylan,

I just replaced my gas tank a couple weeks ago, there was no rust but I sprayed the "original style color" primer and then undercoat spray over that.
I did it because you can't see between the tank and bottom of trunk when the tank is installed any way, but it gave me piece of mind that it would have less problems of rust down the road.


Jason

[DylanFan52]

Thanks Bobby, Lucas, Scott, and Jason!  You gave me some good ideas and suggestions.  I think "piece of mind" describes what I am looking for.  Since I have the tank off I want to address the rust and do something to inhibit more in the future.  Good point in that the car will be driven very little in rain and definitely no snow.  Thanks again!  I am sure I will have more questions as I begin this project.

[55 cadi]

Hello Dylan,

I almost forgot to mention something you will probably need to know.

I started a post a week or two ago about my gas gauge, because I too replaced my gas tank recently.

Turns out that the original gas tank touches the trunk floor pan to make it grounded !

Which is probably why It rusted there is that there is bare metal.

So when you paint/undercoat the trunk floor pan the gas tank is no longer grounded which will make your gas gauge spike to "full"

All you need to do is to ground at the tanks sending unit, put a wire with connector on each end and attach it to a screw on the sending unit and then screw it down to the under body close to sending unit and scrape away paint so it is screwed to bare metal, and then it will be grounded again.

Hope this helps, and before you have it done, sorry

Jason

[DylanFan52]

Thanks so much Jason for thinking of the potential problem with the grounding of the tank.  I would not thought of doing that and probably would not have thought of the solution.  In fact I put a second coat of Rustoleum on today and debating whether to spray undercoat.  I probably will since it is easy to get to.  The tank is due to arrive tomorrow so I am still in the process of prepping under the trunk. Main trunk floor above the tank is solid, though I did find a few repairable rust holes in the trunk floor/back seat area of the floor.  Again thank you.  I am sure I will be calling on this forum for help as I proceed with the restoration project.

John (DylanFan52)