Sadly the metro has a few places which are known for rusting. This can be prevented easily before it starts, or can considerably reduce the rate if there are signs of it starting.
Why does it rust? Rust appears when water is trapped. Water likes to keep mobile so will rust its way out to when stopped. Where mud can get trapped, mud will hold water and this means that the metal work next to the mud spot will over time start to rust. As most Metro's & 100's are over 10 years old the chances are that there will be a lot of road use mud collected in these areas. This means its a good idea to clean out any area where mud has accumulated as this will keep the area dry and without the damp the metal work cannot rust. There's a number of ways to clean the mud away, easiest is with a pressure washer but a stiff brush and a bucket and hose will do, tooth brush if your really keen aswell. Locations of vulnerable points:
The rear arch area is commonly know for rusting as it is easily noticeable as it appears on the outside of the car. To prevent this simply clean behind the arch itself. If access is restricted it will be easier to remove the road wheel. The picture below shows road wheel removed to gain access to clean out behind the arch and other areas. Fronts rarely rust due to having a plastic arch liner fitted, A-series dont have this so same applies to the front arches as the rears.
Rear sub frame mounting points
While cleaning out the rear arches you may as well clean the sub frame mounts also as this is a structural area. Mud can build up hear resulting is the sill rotting and the sub frame mount area to become an MOT fail. Located in front of the rear wheel between the door and rear wheel arch. The picture below shows the sub frame mounts clear of mud build up. The side skirt is visible to the bottom left of the picture to locate this area. The small dots are stone chips and bits or tar which didn't come off with the hose and this area can be difficult to access with a brush.
Petrol tank filler pipes
Same process again, picture below shows the pipe clear from mud build up behind the cap to the tank. Build up here can result in the filler pipe rusting and small petrol leaks or fumes lost and rusting of the bodywork around the petrol filler cap but this is rare but not unheard of so best to clean also.
Rear boot floor corners
The boot floors can rust near to the rear lamp, this is because out build up from underneath the floor of mud yet again. The picture below shows yellow lines where mud can build up so its wise to check hear also, the blue bar shown is the rear anti roll bar.
Sills and floor
Water gets trapped under the carpet and insulation in the front foot wells quite often. The triangle highlighted shows where most of the damp will be found.
To gain access under the carpet simply under at least 4 of the screws which the red arrows relate to and keep folding back the carpet till this triangle area is seen. If damp its best to let it air for a day or more to dry out. If puddles are found there are 2 rubber drain plugs you can remove pictured below. The picture below also shows a damp floor after folding back the carpet back and the 2 grommets removed from the drain holes. Note most of the insulation and padding removed. The foam padding insulation can hold a lot of water if wet.
The picture below shows the left front jacking on my 1.1c model which is identical on all metros. The yellow lines show where mud thrown from the front wheels can get stuck under the car. Its the same for both side of the jacking points and check the rest of the sill also. This area is a very good location for extra protection to be added like under seal.
Another reason can be an excessively perished front windscreen rubber seal, Blocked sun roof drains or the heater intake drain pipe is blocked under the bonnet with a small rubber cover over it.. The yellow arrow below shows the heater intake water drain pipe which on Right Hand Drive (RHD) models uses the left bonnet vent. The lines show where leaves can build up making a kind of dame. Rain which gets into the vent will drain out of this pipe, but if blocked it can over fill and then follow up along of the air route and end up on the floor inside the car. Which will mean a tiny puddle inside on the carpet or even worse underneath and the water will have no choice but to rust its way out. Side skirts can also hold mud and water behind so it may also be worth removing these items and clean and even adding extra protection like under seal to these areas.
Picture below shows a sunroof gutter and one of the 4 drain holes which are in each corner of the sunroof. The tubes run down the window pillars (Front A pillar & Middle B pillar) and out to the bottom of the car.
If major rusting is found, its best to get a professional to sort it as soon as possible as it will only spread so may be come dangerous and will only cost more if the area affected increases.
Also apart from cleaning out mud build up from the car ever so often, it may be wise to add extra protection to any area mentioned with the likes of under seal or similar rust prevention product.