A Guide to Rust Proofing the MGB Roadster by Andy Harkness.
Special Thanks to MG Experience members for their original forum contributions and postings and Skye Nott for allowing distribution of information collected from his great site.
MGB Rust Prevention - Background
Ref: Sam Elter
The "hollow-spaces" you are talking about in the MGB's monocoque chassis are referred to as 'Box Sections', and are rust proofed in accordance with the techniques used and developed by the marine and aviation industries for use in ships and aircraft; i.e. by shooting a corrosion-preventative wax/oil based substance into them using a high-pressure air injection gun with a long flexible hose 45-60 cm in length to facilitate access through strategically placed drainage and inspection ports. Living in the Netherlands, you have a significant advantage over all us 'Yanks' in this regard, as box-section corrosion-proofing is considered just a part of basic car maintenance in Britain and Europe (Whereas in the United States of America Land the preferred method is to just simply replace your sills and dog-legs every couple decades.), so you have access to a large selection of corrosion preventative products designed for this purpose (And any one of them I think you'll find is a superior option to waxoyl!). I would look at the following companies for the information you need:
- Dinitrol - UK Website
- Dinitrol Pricing & MGB Rust-Proofing Chart
- Bilt-Hamber (The winner of many Practical Classics Long-Term Tests)
Hopefully this will help you. From personal experience, I have found that the bottom edge of the front fenders and the rear dog-legs rust-out the fastest, as these sections naturally trap debris and moisture, and the factory allowed no provision for drainage in these sections (Whereas the sills last quite a bit longer as they DO have well-placed drain holes!). So I would remove the front splash panels, and vacuum out the debris collected there, then carefully drill a couple small drainage holes in the bottom of the front fenders, and the bottom of the rear fenders underneath the dog legs. This combined with regular (Once every 1-2 years, though an infrequently driven vehicle can potentially last 3-5 between treatments depending upon exposure) wax/oil injections into the box-sections should effectively keep the corrosion at bay.
Since you cannot adequately prep box-sections, I would strongly suggest that you stay away from spraying any kind of paint product inside them, despite the fact that people do this quite regularly in many classic cars. Because you cannot easily or effectively remove old oil, road- traffic residue, or rust/mill-scale prior to applying the coating you can guarantee yourself a failure of that coating sooner rather then later. Corrosion preventative wax/oil formulations are considerably more forgiving to surface prep, so these are the superior option for this application. May you and your car enjoy many rust-free years!
Ref: Bob Muenchausen
Sill rot depends as much on protection between the outer fender skins and actual outer rocker section (which extends from behind front wheel well to just in front of the rear one). Most sill rot that I have seen over the years occurs in these places much more commonly than in the hollow box section in between. Getting a rust preventative like Penetrol (actually an additive to assist the flow of oil based paints to a smooth finish) between those panels is very important to preventing dog leg and lower front fender rot.
The dog leg area is actually pretty easy to do, requiring only the removal of the upholstered side panels behind the seats. Penetrol (or whatever you like) can then be sprayed into dog leg area between the outer fender skin and the visible part of the outer rocker and let drip out the drain holes on the rocker panel. Up front is a little more involved as it is easiest to do when the closing panel at the rear of the front wheel well is removed.
However, sill rot also depends on how well we seal up the seams and joints where dust and moisture get in, in the first place. The dust that settles in after years on the road is the sponge that holds the moisture in place to do its work. Keep both of them out and you've probably won half the battle.
Ref: Bob Muenchausen
Front or rear, the area between the outer skin of the fender panels and the inner indented extensions of the outer rocker panel are where the rust out begin and continue. Both moisture from road wash and rain while sitting, work their way into any areas where the seams are not well sealed, and then dust - often very fine dust like talcum powder - accumulates between these two layers of sheet metal, acting like a sponge to hold the moisture in contact with the metal longer than if it weren't there. Quite often the blistering rot we see in either area is almost entirely confined to those two panels unless it has gone so far that holes have been allowed to develop through both, and then the moisture can move inside the rocker box section.
The rear often suffers because of poor sealing as much as the front. The front's splash panels and rubber seals do an OK job of deflecting road wash and crap, but I don't think anyone should rely on them alone. I seal mine with a butyl sealant applied by hand. At the rear, there is a small triangular closing panel whose seams often get overlooked or has simply lost whatever seam sealer was originally used. I use the same butyl rubber seam sealer liberally at all the seams in this area (see photo) as it seems to remain plastic enough to seal well over time. I did forget to mention above that sealing along the very bottom edge of the dog leg and also the rear of the front fender panel need to leave the drainage holes open. These closed panel areas have to breathe and drain, or moisture trapped will still do damage, only much more slowly.
You can paint/coat these surfaces as you wish and that is always a good idea. But keeping the moisture and dust out as completely as possible is still, IMO, the best preventative, with Waxoyl, Penetrol, POR-15, etc, all just cheap insurance against what we may have missed or has been burned off via welding as Pete says. In any case, my luck has taught me that there is no one magic bullet to preventing rust in this area, and to think so is a mistake. It is a combination of approaches - painting the panels before welding, waxoyling or painting (to where the waxoyl/paint runs out between the welded panels) and then sealing the seams externally to keep the gross stuff out in the first place - are all requirements, and all good suggestions if used together.
Ref: Ron Lee, South Carolina, USA
I replaced both doglegs on my 67 GT. About a month ago, I sprayed inside the sills and car (behind the quarter trim panels) with Eastwood's Heavy Duty Anti Rust spray (oil and wax mixture). I later noticed that it dripped out from the bottom of the dogleg. Although I mimicked the factory spotwelds when I put in the repair panel, there is still some space between the welds, i.e., it's not 100% sealed. Try adding your rust inhibitor inside the car and see if you get the same result. Might not have to drill a hole.
Ref: Jack Austin, Blowing Rock, NC
As I understand it Ospho is a trade name for a solution largely made up of phosphoric acid, which in turn is a derivative of muriatic acid, the stuff that masons use to clean brickwork, which is in turn a diluted form of hydrochloric acid.
CAUTION! DO NOT use muriatic acid to clean metal parts. The vapors are hydrogen gas and are very explosive. In addition it is hard to neutralize before painting and could cause rust to start below the paint.
In my experience it combines with the surface of a ferrous metal and changes that surface to a hard shell-like coating that defends the underlying metal from contact with moisture and resultant oxidation (rust).
I would think that just about any non-invasive material, such as Waxoyl, would only add to the defence.
Many years ago a business partner and I did a rotisserie resto on an early 911. We cut openings into all of the hidden cavities and poured in Ospho. There was never a finalization to the anti-rust issue as the car was totaled while parked by an inebriated motorist.
Ref: Glenn Towery
I have used Waxoyl for 25 years on alot of my MGs & I am very happy. My 1974 MGB GT V8 has sat out side all of its life & it has O/E dog legs! Oh, she also has 539,121 miles on her today.
I've reapplied Waxoyl every year for 7-8 years & now it is every other year.
Ref: Chris Edwards, Texas, USA
You need to remove or neutralize the rust before waxoyl. Waxoyl is not a rust converter/neutralizer it is just a semi liquid sealant (essentially) that keeps moisture from getting to the metal. if there is rust under the waxoyl, it will remain rust and could continue to progress if it is bad enough. what can seal moisture OUT, can also seal moisture IN...
Ref: Steve S, So Cal, USA
Re: POR-15. I buy it either online or at my local British parts house (can you believe they still exist?) who also happens to sell it.
A pint will give three coats to the entire front end including cross member, with paint to spare. It's super thin stuff and flows well. A common mistake is to apply the stuff like house paint, but that just makes it harder for the POR to cure properly. The first coat should be as thin as you can make it while still getting full coverage.
Marine clean is just a heavy duty degreaser so you don't have to buy it instead of using what you already have. Metal Ready is an phosphoric acid intended to etch the metal and give a light flash rust before applying POR15. Personally I've had best results by blasting the parts clean, then etch with Metal Ready and finally applying POR with a brush. I've had less optimal results by spraying but some have reported good results. Maybe I screwed up the thinning process.
If you don't blast the parts, you do have to make sure they are REALLY clean. POR15 works by penetrating the pours of the metal and bonding to it. The smallest amount of contamination can weaken this bond, and in a worst case scenario it can peel off in sheets. You need a slightly rough surface with no oil, grease or dirt. After etching, I don't even touch the parts with my hands but rather use latex gloves to avoid oils and such from getting on the metal. Overkill perhaps, but I've never had a failure and the stuff has been hard as nails even on my daily driver, so I'm not changing my methods!
MGB Rust Prevention - Problem Areas
Dog Leg and Sills
Ref: Bob Muenchausen
For most rust spots the key words are prep and vigilance. The infamous dog leg panel rot in front of the rear wheels and the rot of the rear of the front fenders at the rocker level both come from inadequate drainage as Peter notes and inadequate sealing of the seams. The front panels benefit from good sealing of the closing panels in the rear of the front wheel wells and an occasional opening up and removing of fine dust and debris that collects between the fender and rocker panels. The debris/dust acts like a sponge for moisture, holding it long enough to do damage and blocking the drain holes.
In the rear, inadequate use of seam sealer between the fender panel and its seams with the rear rocker extension allow road wash to move in between and also get soaked up by dust and debris allowed in by poor sealing. Certainly, use of materials such as Peter suggests will help protect these panels from moisture that unavoidably gets into these areas, but keeping the bulk of the crap and water out of these areas to begin with is also a good preventative measure as well.
Trunk and Gas Tank Area
Ref: Ray Murray, Bellevue, WA
You might discover more rust on the underneath side of the boot floor panel and this is one area that you're "in for a penny, in for a pound" thing. Remove the fuel tank; drain the fuel and sludge in an oil pan and dispose "properly"; let the old tank dry out (or blow it out) and recycle to a metal salvage operator. Chris Frantz has pointed you in the right direction for a replacement tank. That fuel tank is what, 38-39 years old?
You got some pretty good advice on this thread from some very knowledgeable folks, e.g., Bob Paquette= x2; Dick Moritz = x2; Albert= x2 & Purple GT = X2.
After installing the tank, whether new or existing, DO NOT put those rubber strips back on the top of the tank. You will notice that all the rust, or the most rust, has accumulated mostly where those inferior British rubber strips were located. Instead, use a good quality silicone sealant and place a 1/4 inch bead about 3/4 inch from the edge on the top of the tank. Make doubly sure that there is a generous amount of silicone sealant on the lead edge of the tank where it joins the floor of the boot. All the spray and road debris tries to collect between the top panel of the fuel tank and the boot floor panel. By making a moisture proof seal in this area, your fuel tank will last for your entire life time. This has been my experience on 4 MGB fuel tank installations.
Commercial Vendors and References
Electrolytic Rust Removal
Videos - Rust proofing applications
Rust Proofing Equipment
Home Made Sprayer Systems
Ref: Paul Gurnee FerndaLe, CA, USA
- Purchase a 1/2 Gallon garden sprayer.
- Remove the wand.
- Push a 1/4" ID black vinyl hose over the threads. The vinyl is soft and seals around the threads.
- Insert a 3/8 OD-1/4 ID poly tube into the black vinyl.
- Insert a 1/4 OD-.170 ID poly tube into that. The poly is stiffer than the vinyl and is easier to push into the cavities.
- Thread a drip irrigation tip into the 0.170 poly. I used the 12 gallon per hour red tip with a 360 spray pattern.
- I filled with Ospho first. Pumped 15 times & Sprayed.
- Let it dry.
- Filled with Penetrol. Pumped. Sprayed.
Home Made "Waxoyl" Recipe
It's an old fashioned rust treatment and undercoating:
- 2 1/2 quarts turpentine
- 12 oz. beeswax / candle wax
- 1 quart light machine oil
With a cheese shredder, cut the wax into the turpentine, stir until the wax has dissolved, (takes a long time; you can use very low heat (a warm room) to aid but be careful) and thin with the machine oil to a brushable, sprayable consistency. Apply liberally. You can use a hand spray bottle to get into closed-off sections if you have a small access hole.
Please be sensible when you make this stuff; don't go breathing the fumes or applying heat and burning down your house. If you have any doubts about it, err on the side of caution and just buy a commercially available product.
Where to Apply Waxoyl or Other Rust Proofing
Lindsey Porter's book Improve and Modify the MGB (available via the MG Experience Store, worth buying) has a few pages on rustproofing.
Here's where he suggests spraying:
- Underneath the chrome trim strip below the rear windows on GT's
- The 'ledge' inside the front wings which can be accessed from inside the engine bay
- The tailgate doors on GTs and the boot lid on the roadster -- spray inside the small box sections
- Inside the doors (remove the doors cards, obviously). Clear the drain channels if not open.
- Beneath the quarter lights inside each door (water enters through dried gaskets)
- Inside the front box section of the bonnet (there are holes)
- Clean first, then spray the front inside of the fenders (I also spray the back of my chrome bumpers since they get rusty back there)
- Remove the vertical mud trap panel from behind the front wheels, and spray there -- or drill holes into the vertical panel (plug them later) to do this.
- Drill holes in sills below doors and rustproof the box sections (as I mentioned above) -- and remember there are two parallel box sections
- Where the rear inner wing joins the rear outer wing is a rust area. Spray from inside the trunk (remove any trim from inside the trunk to access this)
- Main chassis rails -- there's a hole at the rear giving some access
- Center chassis rails through holes available
- Through holes in the smaller 'castle' sections alongside the frame rails -- there are pre-existing holes
- Inside the cross member which joins the two rails
- The support for the rear spring-shackle where it fits against the body at its forward end has a rust-prone seam which should be soaked with rustproofer
- Jacking points