Rust Detection, Prevention and Repair for Classic Minis

Rust and Minis have a great affinity for each other. Sooner or later all owners of non-restored old Minis will have to tackle this problem. This article covers rust detection, prevention and repair.  With production running from 1959-2000, your Mini (or the one you are considering buying) is no longer a youngster, so inevitably rust will be an issue to consider in your plans for the vehicle.  At Mini Mania we have nearly 45 years of experience in renovating Minis and providing parts to repair them in a timely and cost effective way.

Rust Detection
Here are the locations you should check out on your vehicle:


1. Headlamp surrounds and bowls. These are part of the front wings. These rust out from behind due to accumulation of road dirt and moisture, usually only in locales where salt is used on ice.

2. Lower front bumper lip. Often curbed and hit by stones, and therefore prone to rust where paint is chipped away.

3. Lower windscreen surround, top of wing, top of a-panel. These areas rust because of the box section inside this area, which accumulates road dirt and water. The rear seam of the bonnet also rusts because of the foam soundproofing it sits in that retains water.

4. Lower edge of A-panel. Rusts out from the inside.

5. Roof skin edge / cant rail. A trap for dirt and rain, prone to rust from the outside in as paint wears. Rust is accelerated greatly on Mk1 cars with sound proofing foam inside the cant rail.

6. Rain channel (Mk1/2 cars). Does not drain properly, by design.

7. Door lower edge. Rusts due to blocked door drainage holes.

8. Inner wing / front of sill. Very prone to rust from accumulated road dirt.

9. Outer/inner sills. Very prone to rust from road dirt/water trapped inside, and paint chips.

10. Floor pan, particularly the front quarters.  For those intending to restore a car, patch panels are available for most common rust areas.

11. Rear subframe. A common casualty of inadequate factory rustproofing. The heelboard is also likely to be rusted.

12. Lower edge of body sides. More common where road salt is used.

13. Boot lid. Prone to rust along the bottom edge. The same applies to van/estate rear doors.

14. Rear panel / rear valance / rear bumper mounting flange and boot floor.

NOTE: Rust may also occur where crash damage has been improperly repaired and covered over with filler.

Types of Rust
There are two major types of rust.

First, "surface rust" or rust that works from the outside of a panel inwards. This rust starts when paint wears or chips away. Depending on environmental conditions (humidity, temperature and use of road salt), this rust may progress at a slow rate and can often be sanded or blasted off. This type of rust is usually found in the roof channels and external body seams, and where the paint has become chipped. This rust can be prevented from developing into a problem if it is sanded out and repainted - if your car has any rust like this, treat it now.

Second is rust that works from the inside out. This is usually characterized by loose rust flakes and bubbles in paint work, and has worked its way to the outside from the other side of the panel, e.g. from the inside of the A-panel to the outside. This type of rust is far worse and is more common in countries that put salt on the roads during winter. The only hope for repair is cutting out the rust and replacing it with new metal.

If a car has rusted to the extent where holes can be found in any of the above locations, heavy-duty repairs are needed. A car with rust in the roof pillars, roof skin, floors, or worse, should be avoided unless it is a collectible model or intended for restoration.

Beware of a car with fresh paintwork. It can take from six months to one year for painted-over rust to bubble out again. Meanwhile, it has been spreading under the paint causing severe problems that may not be detected until it is too late. Stay away from any car with blisters in the paint as it is a sign of hidden problems and poor paint preparation work.

Rust Prevention
If your car body is in good condition, rustproofing it is an effective means of keeping it that way. Minis have until recently been delivered with almost non-existent rustproofing. It is not uncommon to find the insides of sills and other box sections to be bare-metal, having not even received a coat of primer. As the car was assembled before painting, the inside of the door hinges is usually bare metal.

Undersealing the floor and wheel arches is an excellent method of preventing rust. It can be applied professionally, or can be purchased in a form that can be painted or sprayed on. It then dries into a tough, flexible coating. The areas to be undersealed must be absolutely free of rust, or the underseal will allow it to spread while hiding it from observation. Car must be taken not to paint shut any drain channels such as those in the sills.

Box sections, such as the sills and floor cross member can be protected by injecting rust-proofing wax into them. The wax/fluid is applied with a special nozzle designed to reach into enclosed areas through a small opening. For best results the wax is heated to thin it, and after application it sets into a moisture-proof layer.

Small crevices and awkward areas, such as the drain channels in the doors, and the external body seam covers may be protected by applying deodorized fish oil or a similar oil designed for the same purpose. This oil is usually provided in a spray can and has the ability to creep between welded metal flanges to protect otherwise-unreachable areas.

To delay the onset of rust, keep your Mini clean and dry and ensure the underside is regularly cleaned.

Rust Repair
The most important principle of rust repair is to catch it as soon as possible. The cost of repair increases dramatically as rust progresses from being superficial to structural

Superficial rust can be removed by several means. The most convenient is bead/abrasive blasting, which will remove all rust and paint with minimal effort. Providing no holes were revealed, the surface is then ready for rustproofing and painting. Other methods are by hand sanding or machine grinding (use 80 grit sandpaper and finish off with 120 grit to remove visible surface scoring), use of a Sand-n-Strip disc or acid dipping. See our body stripping article for more details.

The various rust-treating gels and fluids do not give very good results. It is better to remove the rust by abrasive means and then use metal conditioner to prevent rusting, as the rust-treating products are essentially the same as metal-conditioner. If you use them, use the type that dries to a harmless coating rather than the type that must be washed off, as this type is liable to become trapped in body seams from where it can leak out onto your new paintwork.

If your car has surface rust, the time to fix it is NOW. At the least, use a Sand-n-Strip disc, grinder or hand sanding to remove the surface rust. A wire wheel may be used to get into inconvenient areas such as the roof gutters. If you do not wish to remove all the rust, or cannot get at it, use POR-15 or similar "Paint over Rust" type paint - use only a recognized brand name as many of these paints do not live up to their claims. At the least, apply metal conditioner and primer, followed by top coat as soon as possible. Even if the results looks ugly, the rust problem is reduced or eliminated. If your car is better than a tatty runabout you may desire a better quality finish, in which case it is best to leave the painting to a professional

If the rust is more than superficial, it must be cut out and replaced with new metal. Either the entire panel containing the rust should be replaced, or a patch several inches larger in diameter than the rust should be welded in. This ensures that no smaller outcroppings of rust remain in the panel.

Mini body panels such as the wings, sills and front panel, which often rust, are available so cheaply that it is not worthwhile to patch them. Panels are spot welded together; these spot welds must be drilled out for the panel to be removed. A new panel may then be spot or MIG welded in place. If you are paying someone to do the work for you, it will be cheaper to have the panel replaced than patched

Patching is a skilled art. It is easy to weld a patch into a piece of metal, but very hard to do so without causing welding distortion which is hard to straighten out, especially if gas welding is used. However, it is far more practical than replacing the entire floor pan or a body side. Pre-made patches are available for most common Mini rust-spots

When repairing the floor and sills, it is possible to remove too much metal causing the body shell to twist. If you intend to remove both outer sills at once, or more than half of the floor pan, the car will need to be jigged or at least have its weight supported by bracing.

The need to do bodywork when restoring a car is caused by rust 99% of the time.

Mini Cooper Sheet Metal