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MG MGB Technical - Rust treatment trunk floor/gas tank

In the process of installing a new gas tank I have cleaned the underside of the trunk and found some rust through in the floor under the spare tire bolt down. I am considering using a hole saw to remove the rusted floor area exposing the bottom side of the bolt down and then treating the underside of the bolt down with penetrol.
Also, how have you all treated the underside of the trunk before reinstalling the tank?
Any input would be appreciated.

Regards,
Dennis

PS The car has limited rust areas but I suspect those who advertise original cars without a speck of rust haven't looked in some of the areas I have been working on lately.
D F Sexton

If I take my tank out again, I think I will cover it with por15 and dynamat like I did my 53 Chevy.
kids1

I have recently discovered this:
http://www.masterseriesct.com/
Sold by a longterm VW restorer, said to be the best and cheapest and easiest to use of all such anti-rust paints. Nice guy and fast service. I will let you know in a few years how good it is!
I use wax based rustproofing from JCWhitney, and their $40 gun to put it on, or brush on for small jobs. Paint the floor and the top of the g tank. Works absolutely great.I thin 20% with motor oil for inside boxes, etc.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Some years ago, I used a product called "Extend" to kill and convert rust into a paintable coating. It seems to have worked very well at what it claimed to do and it has resisted further rusting after 15 yrs continuous use as a daily driver and exposure to all the normally corrosive elements we drive in.

However, I noted that it did not fare as well as some other products mentioned in this site's review of rust treatments, so I offer this URL up as a contrast to my own car's experience: http://www.geocities.com/goodytommy/rust/ FWIW.
Bob Muenchausen

Bob, all manner of people have tried to convince me of "Extend" benefits, but everytime I use it, the results are almost as good as bare metal and slightly better than painting with battery acid! Everything else I have ever done works better. Is there some sort of secret??
FRM
http://www.usachoice.net/gofanu
FR Millmore

Bob, looked at Extend, the caution says do not use on unrusted metal as Extend may cause rust so I passed.
What I have done -
- used a 3 1/4 inch hole saw and cut out the bottom of the spare tire hold down, wire brushed out the rust, treated with penetrol, caulked the small openings and topped with two coats of rustoleum.
-painted the tank with multiple coats of rustoleum.
Plan to use the rubber packing strips -- one side to side at the leading edge and the other three front to back as they were originally. Then plan to put a rope of window caulk around the entire perimeter of the tank just where the top welds to the bottom -- my thought is to prevent water from getting between the tank and the trunk floor.
This has taken longer than the typical 3-4 hour estimates because of cleaning the soft undercoating from under the trunk and painting the new tank -- but I am very pleased with the results.
Regards,
Dennis
D F Sexton

DF,
Do not do it this eway!
Water will get in past the caulk, and will not get out again, until it eats through the tank and falls inside. Rustoleum primers work quite well, and the topcoats are tough too, but not impervious to standing water and especially trapped vapor, which is what the caulk will cause. You have probably sealed up the weldseam with the paint, but not perfectly. I always spray the seam with WD40 or similar to get it wet inside, then put oil on the seam, turning the tank to be sure it runs all the way around. Repeat several times. The wetness from the WD40 sucks the oil in. Once there is oil IN the seam, water can't get in. I would run a razor blade around the seam, then do the oil trick. On the top of the tank, it will last a long time if you only use it in the summer, since any water that gets up there can get out again. The problem is that eventually dirt collects, which holds the water. The wax based rustproofings will completely prevent either floor or tank from rusting out ever.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I don't know anything about this, or dams for that matter, but it looks to me like the rubber packing strip which runs side to side would be a good place for a water trap.
Glenn

My $.02 worth. I'm in the middle of a complete redo of a CGT which hasn't been on the road for 20 years, so everything needs doing! When I dropped the gas tank what I found was a rust flaking mess, but only the tank, the floor above it was nearly perfect! Explain that! Now to replacement precautions. I bought a new tank and degreased the outside, then two coats of Rustoleum primer(I like the stuff), then I used the spraycan version of truck bed liner to coat the entire tank. It dries tough and hard(and it's cheap!),and gives an interesting semi-pebbled surface. I reinstalled with doubled packing strips. You have to let trapped moisture out or any anti-rust treatment will eventually fail.

Paul Briggs
Paul Briggs

FR, etc:
Why do you think I posted the contradicting info from the website? Let the buyer/user beware of any number of wonder products.

I have found that in most things, experiences can differ with any given product or means or methodology. That just seems to be the oneriousness of life in this universe. What works for me may not for someone else and vice versa.

Not being a chemist or metalurgist, I can only tell you that in 15 yrs of service, the use of the Extend material has not seemed to have done what you say it should have by now. So either I am damned lucky (that is some folks opinion) or perhaps we don't really know nearly as much as we think we do about the predictability of the things we use and create.
Bob Muenchausen

FR:
That is a good trick you recommend to DF and I would add another possibility.

The packing strips between the tank top and the underside of the trunk floor are shown in factory drawings to be installed across the top to the tank from front to rear. This causes them to trap water, dirt, etc between the packings in the depressions formed between the ribs of the tank top.

When I replaced my tank, I chose to run the packing strips along the tops of the ribs from side to side. The hope here, along with having used a good quality epoxy enamel coating on top of the tank, was that there would be better drainage, but more importantly, better air circulation in these spaces between the ribs, since those were the areas which had the greatest rust on the old perforated tank.

The combination of trapped water and poor air circulation seem to undo the best laid plans and products of man, if given half a chance.
Bob Muenchausen

Paul B's experience is due to just the right amount of dirt to keep the tank damp, but not the floor, coupled with chemical reactions with the terne-plate of the tank. A teaspoon of water trapped is much more lethal than full flooding; the water is a catalyst for rust, but oxygen is also required, which liquid water only has a small amount of. You can observe that on a partially submerged object in still water, the area near the water surface is severely rusted, but below the surface it is moderately rusted. Like when your shop roof leaks into containers of precious parts!! The problem with any "hard" or impervious film, as truck bed liner or undercoat, is that it traps that little bit of water, while allowing oxygen to enter. Modern paints are very tough, but when perforated, water gets under them and sets up the same condition, leading to the progressive circular rust tumors we all hate. This corrosion is much more severe than that of a plain bare spot. The wax based materials stick to the surface and are self healing. It is arguable that it is in fact preferable to apply the wax to bare metal. That is a little much for most of us, so we paint and use the wax to seal defects in the paint film. If you use the wax based rustproofing, let it set up a bit, then stick the packing strips in the wax. Bolt it together for the LAST time! Use antiseize on the bolts.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Wax alone protects bare metal very well. I keep a coat of paste wax on all my tools, especially things like milling machine chucks and lathe beds. Also on the saw tables. Most of this stuff is cast iron and stays shiny as the day it was new. No rust whatsoever, even in the heat and humidity of the gulf coast. That's what makes products like waxoil so good.
R. L Carleen

Well it's installed and the car is back running.
-cleaned all the undercoating off the underside of the trunk floor
-painted the new tank with multiple coatings of rustoleum
-used paste wax over the entire tank
-coated the top of the tank with penetrol
-turned the tank on edge and worked penetrol in between the top and bottom of the tank along the weld seam, there were a few spot welds but the penetrol seemed to work completely around the tank
-the original packing strips were placed one side to side at the front and three front to back, but I cut the new ones to length and placed them side to side on top of the ribs on the tank, using another coat of penetrol to stick them down
-coated the underside of the trunk floor with penetrol
-reinstalled and back on the road
Now I can try to get back to other cleanup, tune up and rejuvenation projects on the
car.
Thanks again for all the time and advice.
Regards,
Dennis
D F Sexton

FR makes an excellent point about wax vs. paint. In the paint business the microscopic imperfections in coatings are called 'holidays' and are always present. Corrosion starts there and perforation can be quite fast if the new metal is trying to 'protect' the rest of the car. Our cars tend to have plenty of surface rusting at the same time so the corrosion effect is spread out and much slower overall. Short of cathodic protection with sacrificial anodes or an impressed voltage(or both), rust somewhere is inevitable. That reminds me, I have some cold galvanizing compound somewhere in the shop and I think I will slather on a good coat in some inconspicuous place to protect the car for awhile(and maybe drop the tank again and waxoyl the top)

Paul Briggs
Paul Briggs

Sounds like DF did the right things. RLC, you have helped me out - despite my dissertation re wax, I never thought to wax my machine tools. I have literally watched the lathe and mill rust before my eyes, in a matter of minutes, when high humidity moves into the cool shop in the summer! Thanks! Paul B set me to thinking about impressed voltages - it is common for the fuel pump and fuel gauge grounds to be attached to the tank, which is indeterminately connected to the car, possible reason for tank death.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I fitted a stainless tank in 88. And took it off again to have it welded in 2000. There was some surface rust on the boot floor , but nothing serious. I repeated the treatment as before ,wire brush "Curerust then hammertite then waxoyl.
The commnents re the rubber spacer strips are interesting , a thicker one might keep the place a lot dryer .
S Best

I agree with FRM, Masterseries makes great paint, especially their 2 part paint which is as tough as nails if applied correctly. I plan to paint my floors today with their black semigloss and it IS resistant to sunlight. I used it on my front suspension and other parts last year and it still looks like the day I painted it. I also bought a gallon of truck bedliner paint and used that on top of POR15 and it looks great as well. I also bought the JC Whitneys rustproofing gun kit and found it to work great although I did use Waxoyl with it. I did that messy job yesterday.

I plan on waxoyling the floor where the tank is although I did heavily coat the area with POR 15 already and I plan to put Waxoyl on the top of the tank after I strip the tank and have a friend shoot it with some 2 pak paint.

FWIW I use waxoyl on all the bolt threads so they don't rust up. I also use antiseize too at times.
Mike MaGee

All the comments on air contributing to rust generation are, of course, right on. However, one thing that many of us have observed is that when moisture is trapped in pockets of accumulated road dust, such as on top of a tank or between the bottom of a front fender and the outside of the hidden front end of the rocker panel, rust seems to be accelerated by being held in place by the "mud" produced when moisture is added. Some of the rusting action is perhaps caused by misc chemicals (some of which may well release oxygen on their own when mixed with water) present in the mud and some is probably from the air.

My thoughts on providing ventilation in these areas was not to provide additional air to fuel the rusting (I would think that any reaction only uses as much as is needed) but rather to remove the moisture as quickly as possible, hopefully staving off the start of the process in the first place, and, at the least, reducing the opportunity for it to fester over time. The conditions which seem to produce "festering" seem to be more destructive, more immediately, than whether the metal is simply exposed to air or water, not that those things aren't detrimental as well.

It is an interesting thread, and every time this topic comes up, there are a rash of cures which surface because of folks satisfactory experiences with them, and also a number of different approaches to resolving the problem based on our understanding of physics and chemistry, many of which are indeed excellent and well thought out.

Speaking solely for myself, I tend to look at the long term results for verification, and as we have seen over the years here on this BBS, sometimes they can be contrary and seemingly arbitrary, compared to our science. I suspect that a lot is contingent upon the care given to a car and to the local conditions it is exposed to. Certainly, I would not expect my poor ol' GT to have survived so long nor so well in NJ as it has here in much drier and relatively road salt-free Idaho. I would bet that some of that salt finds its way into road dust as well as into splash of road water, even weeks after the snow has melted and gone away.

Bob Muenchausen

This thread was discussed between 29/05/2004 and 31/05/2004

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