MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Sill Drain Holes

I've recently found a 'filler and ally plate' bodge on my OSR wing and sill, so I made a couple of incisions today to see what I was dealing with.

I was surprised to find a drain hole in the back end of the sill, similar to the two holes below the door, but there is no hole in the lower wing panel.

How is the water supposed to get out?

You may think that I've posted the photo upside-down. It's actually the car that is upside-down, the photo is the right way up! ;o)


Dave O'Neill 2

I don't think water is supposed to get in there in the first place! But I know it does and the cars do rot here, my son's otherwise solid GT had rot just here.

It's not a hard bit to patch, particularly if you've got the car upside down. We have sprayed it liberally with cavity wax, inside and outside, in the hope that it will last for the next few years.

Mike
Mike Standring

There are (or should be) a series of gaps along the seam where the outer sill joins the inner sill for draining the outer cavity. there are also a series of holes in the castle section that drain the inner cavity.

There may well be no visible holes by the front and rear wing cover panels, but is the cavity is continuous the length of the car they will drain from the visible ones.

Water can get in for all sorts of reasons, including condensation forming.


Paul Hunt

Mike

Yes, I think it should be fairly straightforward.

I've cut out the infected bit and the rest of the structure is sound.


Dave O'Neill 2

Paul

My sills don't have those pressings in them. They may have been a later development.

I have two holes in the section below the door, one in the affected area, and I'm guessing that I'll find another one when I remove the front wing.

My original question was as to why they would put a drain hole at that point in the sill when the only place any water can go is into the void between the sill and the wing.

The question now is whether to weld up that hole, or drill a hole in the bottom of the wing to aid drainage?


Dave O'Neill 2

Or a replacement sill that didn't have them, although I'm pretty sure mine are themselves replacements before my time.

Are your wings and sills original? Maybe there was originally a hole up through both panels. But in my experience Waxoyl on a hot day runs out of the nominally flat joint between wing and sill anyway, AFAIK it was spot-welded not seam welded.

Drilled and painted i.e. properly protected before fitting is probably going to be better than filling it in. But as you brake and accelerate any water that is in there will tend to move forwards and backwards, and hopefully run out of one of the other holes anyway.
Paul Hunt

The wings and sills are all original.

I think drilling the bottoms of the wings - front and rear - is probably the best option, followed by copious amounts of Waxoyl or similar.

There was some evidence of silt in the gap between the wing and the sill. I don't know how it got in there, unless it went in through the forward drain holes and came out through the rearmost one.

If you look at this photo, you can see where the rust is spreading backwards and upwards from the drain hole.

Once in there, it had no way of getting out.


Dave O'Neill 2

Dave

I would not advise drilling any holes in exterior panels as they will inevitably present rust concentration points. Holes in sill and castle rails work because they can be copiously coated with thick paint and wax without impacting the appearance of the car.

Paul's advice on generously applying waxoyl at summer temperatures is probably the best advice.

I've been working on sill replacement on my Mk1 GT and am currently researching a good paint that will enhance rust prevention before Waxoyl coating. From what I've read, the problem with traditional Zinc oxide undercoat paint is that it only works where metal is abraded by a blasting medium such as sand or bead.

I've considered exterior paints such as Triflow, Hammerit, or Rustoleum but am not sure if these are sufficiently flexible to withstand panel movement due to (a) temperature cycles, or (b) driving induced body stress.

Can someone please advise on an effective paint brand or type that can be brush applied over either new panel black paint or existing panels that have been cleaned by grinding or polycarbonate discs.

Regards,
Brian McIlvenna
Brian McIlvenna

I would not advise drilling any holes in exterior panels as they will inevitably present rust concentration points. Holes in sill and castle rails work because they can be copiously coated with thick paint and wax without impacting the appearance of the car.

Paul's advice on generously applying waxoxy at summer temperatures is probably the best advice.

I've been working on sill replacement on my Mk1 GT and am currently researching a good paint that will enhance rust prevention before Waxoyl coating. From what I've read, the problem with traditional Zinc oxide undercoat paint is that it only works where metal is abraded by a blasting medium such as sand or bead.

I've considered exterior paints such as Triflow, Hammerit, or Rustoleum but am not sure if these are sufficiently flexible to withstand panel movement due to (a) temperature cycles, or (b) driving induced body stress.

Can someone please advise on an effective paint brand or type that can be brush applied over either new panel black paint or existing panels that have been cleaned by grinding or polycarbonate discs.

Regards,
Brian McIlvenna
Brian McIlvenna

Brian

I'm convinced that the corrosion shown above, together with corrosion in the bottoms of both front wings is due to there being drain holes in the bottoms of the sills, but not in the wings.

I think I probably will drill a single drain hole in the bottom of each wing, but it will be treated to prevent rust.

With regard to your question about paint, which part of the car are you talking about? Is it the underbody?

On the underside of my car, I have used two coats of Bondaprimer, followed by chassis black enamel, all applied by brush.




Dave O'Neill 2

Dave,

I'm thinking of the inner sill membrane, enclosed sides of outer sill, castle rails, and floor cross member; also the floor panel and inner sill step within the car interior.

Thanks,
Brian
Brian McIlvenna

Even if you don't drive in the wet, the MGB will sweat internally. You need drain holes in the wing behind the front wheel and in the sill in front of the rear wheels plus as much waxoyl as you can get in without blocking the holes.
With regard to Brian's question about coatings - Zinga is most effective. The bare metal does not need to be shiny bright but it does need to be free of loose rust and very clean. Galvafroid is an alternative but I can't make it stick as well - tends to dry and flake with time.
Roger W

Roger,

Thanks for the advice on sill paint.

I appreciate that some means of escape needs to be provided for water in the sill and front wings but feel that drilling holes is not a good solution. Could a channel be ground into the middle sill or a concave indentation be introduced on the front wiing? This would obviate the need for drilling holes in the outer sill & wing and thus eliminate the rust formation caused by panel holes. I'm sure some members have good experience in this and can maybe advise further in this thread.

Brian McIlvenna
Brian McIlvenna

Brian - Your point about the risks of drilled holes is well taken - some solutions are more elegant than others but the key is to prime and paint after the holes are made.
For info my wings are one of the exhaust routes from my very hot engine bay - the drain holes let air out as well as water.
Roger W

Roger,

Your exhaust route sounds interesting and a good piece of engineering.

I've been following up on your Zinga galvanisation suggestion. As I discovered from Jordan, the very helpful and well informed zinga UK representative, this treatment has been used successfully for 30-years in the shipping industry. He spoke of the need for a very particular metal preparation profile. Thorough sanding is insufficient; he specified bristle blasting and recommended the MBX air powered unit (www.monti.de).

I'd be interested to learn of anyone's experience or suggestions for metal preparation for Zinga galvanisation.

Thanks,
Brian McIlvenna
Brian McIlvenna

Brian,
I am sure you have been well advised about preparation. Terminal tinrot is a major problem at sea and downtime for repair is ultra expensive, so the original application has to be under ideal circumstances. My experience (of the naval chipping hammer and wash with thinners approach) probably does not pass the most stringent test but it is very reassuring to see the coating intact and apparently unchanged after two or three winters on the road. Despite all that is written here on the web, there is nothing better short of hot galvanising.
Roger W

Roger,

Did you go down the route of applying a paint finish or tar free polyurethane coat on the zinc layer?

I'm considering doing the entire underside and wheel guard areas with this cold zinc coating process. I'm interested to learn of the navel chipping approach. This sounds very slow and intensely labour intensive...

Thanks again,
Brian
Brian McIlvenna

Brian,
Our cars are a constant work in progress so everytime I need to work in an area, I attack anything that looks like rust. The result is that I have tried several approaches with Zinga; the very rusty floor of the roadster has had two coats and nothing else under the carpets - it is grubby but rustfree now three or four years on, the headlight ring inside the wings was rusty under what remained of the schutz - that had two coats of Zinga - a wash with the stuff you coat your galvanised garage door before the primer - then red primer (because I couldn't find yellow chromate) and finally underseal - I don't expect to look at that again, whenever a seam is exposed it gets the same treatment except on the visible coachwork where I cover the Zinga with cellulose undercoat and twinpack colour/shine.

The naval approach to painting is to remove all the old paint with a Vee shaped pointed hammer (aka "chipping hammer"), clean with white spirit and apply "red lead "(admar) before battleship grey. The hammer is important because it exposes shiny metal for the Zinga to bond and work its catalytic magic. The hammer on our MG fleet is an old chisel. On new metal, one simply scuffs the surface with emery cloth.

If you coat the underside of your B and leave it exposed, you will get chips (which) are easily touched up but you will be amazed two or three years down the line.

Keep us briefed !

R
Roger W

Hi Roger,

Thanks for the detail on your experiences with Zinga and rust proofing.

Do you think emery cloth, or fine sand paper, is sufficient to provide a base for Zinga?

Could you advise on a grit grade? The Zinga UK technical support agent insisted that nothing short of grit or bristle blasting would do.

Would the factory black paint on new body panels needs to be fully removed before surface preparation?


Are you advising the use of underseal on the car underside and inner wings to avoid stone chip deterioration?

I'm looking into purchasing or hiring a bristle blaster to prepare the new and existing sill sections, and later the car underside and inner wing areas. It obviates the need for difficult and expensive transportation to and from a grit blasting site.

All comments and suggestion are greatly appreciated.

Many thanks again,
Brian McIlvenna
Brian McIlvenna

Brian,
I really think a medium grade emery will do the trick but the cleaning is really important. You can get Zinga thinners at some cost which is probably ideal but I sometimes use cellulose thinners.

Your question about the factory primer is interesting because any welding will burn it off and create a rust zone. I suggest the correct answer is to remove the primer from the weld line and to Zinga the weld before any other coating is applied.

The question about underseal can only be answered by yourself. If you are prepared to inspect and touch up at regular intervals, the answer may be to leave it uncovered. After all that is what the Germans now do with their suspension parts. If you have time and patience to cure and prime it, then apply the normal coatings. Start with oil-based marine red, if you can get it. Anyway this is a decision you can defer - additional coatings can always come later.

It is interesting that the chips do not immediately rust, they get some protection from the adjacent Zinga.

The cost will make you blanch but the value will become apparent with time. The process has allowed us to use our cars throughout the year - now for many years.

R
Roger W

Roger,

Many thanks again.

I note you indicate removal of new panel factory paint just local to the weld areas; would it be more prudent to remove the factory coating and recover over the entire areas of new panels?

I suspect the almost 100% zinc content of Zinga makes for excellent electrical conductivity, and therefore facilitates MIG welding. Am I right in assuming this? Before learning of Zinga, I had been considering using the U-Pol aerosol weld-through primer to prepare welding areas.

I'll need to consider the full cost of the bristle and grit blasting processes before I decide on the cleaning method. I've cleaned the underside before using a scraper to remove underseal and rust; this was done on my back and it was torturous and lengthy working above my head. Bearing the cost of a rotisserie is an obvious solution.

Thanks again,
Brian McIlvenna
Brian McIlvenna

Brian

I used a rotisserie and even then it was a long, dirty job.
I can only imagine how bad it would be lying on your back.
Dave O'Neill 2

Brian,
It's the weld areas that rust and therefore need protection. I doubt the Zinga would hinder the weld process but I am not sure what the heat would do to its effectiveness. Since we know that the coating will work on a clean weld, it may be best to apply it after the weld and yes, I expose the area around the welds but not other areas where modern paints and or waxoyl keep rust at bay. With the exception of one very small area, the Zinga is on the underside or inside our cars.
Waxoyl (or supertrol/Dinitrol) are an important part of the war on rust as is the spoiler and oversize mudguards on the front wheelarches.
R
Roger W

This thread was discussed between 15/06/2014 and 03/07/2014

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.