Most modern cars are based on a monocoque this means the body of the car and the chassis are one component rather than the body on chassis design on older cars.
If rust gets into the monocoque it can weaken the structure of the car and can ultimately make it unsafe to drive, which no one wants! From factory the lower exposed parts of this monocoque are protected with underseal which acts as a defensive barrier against dirt, rocks and more importantly, water and road salt.
Over time that barrier of underseal begins to wear away. Luckily a company called Hammerite produce a substance called Waxoyl which you can use to reundercoat your car! You can buy yours from Amazon here: Hammerite Underbody Seal with Waxoyl 600ml Aerosol
This comes as a spray or you can buy it so you brush it on. It’s really easy to do as is a great piece of preventative maintenance that will stop you from potentially having to spend money on welding rust holes shut at a later date.
First thing you will want to do is get your car up on jack stands and take the wheels off.
Welcome to the wonderful world of my inner wheel arch!
What we want to do here is coat the aforementioned monocoque of the car. Try not to spray any steering or suspension components. Under the arch you will also find plastic arch liner. It tends not to fit amazingly and can be a trap for water, so it is best to take this arch lining off so you can get plenty of protection behind it too.
On a W202 C-Class the arch liner is held on my a handful of these plastic 10mm bolts.
Once you peel away the protective cladding chances are you will be able to see some of the original undercoat. See how nice and black this is in comparison to the grey under the car.
Before you get to painting the new underseal on you want to make sure the car is clean so you aren’t just painting over dirt. Get a nice thick brush and a little bit of water and remove any loose dirt, dust and rust etc.
Then you can paint it and put your arch lining back on!
Underneath is easier if the car is on a ramp but it’s doable on jack stands.
Again just make sure that everything is clean before spraying and when you do spray the underside of the car make sure that you get into all the nooks and crannies as that’s where the rust will take hold.
If you were doing a full restoration you could strip the car right back, take all the running gear, exhaust and brake lines out then do it. Maybe that is something I will be able to do one day but not when I need the car for work the following day haha!
And that’s all there is to it! Nice and easy and it will prolong the life of your car and/or will save you on some repair bills later down the line. It’s an easy piece of preventative maintenance that we can all do!
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