That area seems to be a reasonably frequent problem, and the only way to tackle it is to scrape the black sealant off and clean right back to bare metal with a grinder (fitted with a fairly coarse flap disc is ideal), then a harsh going over with a wire wheel to try and get as much out of the rust pits as possible (wear eye protection for both!).
Then thoroughly soak the area with a rust treatment - I use Kurust - there are other products available, but the better ones are a lot more expensive, so it's down to how much you can afford/want to spend.
Once the rust treatment has dried out, a couple of coats of anti-rust primer and a couple of coats of a decent quality paint (there are any number of epoxy chassis paints available, which is the type I would recommend. You can then apply a good layer of underbody seal to further resist attrition from flying stones and wayward trolley jacks if you wish.
I would not recommend simply coating the bare metal with underbody seal - that stuff isn't as waterproof/airtight as you might think, and in my past experience of it, the rust simply re-develops beneath it even faster than before.
Thanks. The good news is you say it is well known fault and imply it tends not to need welding?
I have a grinder and drills - which I very rarely use. I have never used the grinder to remove rust. I just use the grinder with discs to cut metal. Also a grinder seems more noisy. I have always used a sanding disc on a basic electric drill. Will the electric drill be OK? Or is the extra power of a grinder important?
I am dreading getting the black waxoyl off. I know Hammerite thinners or petrol breaks it down quickly, but still not easy over these kind of areas. Any tips for getting waxoyl off?
thanks for this. How much is the welding? Or did you scrap the car?
Regards the cause, I noticed that in winter this underside area stays permanently very wet. At no time in 24 hours is this area dry. As if the rain water drains are wrong. So that water collects only on these points.
Did the repairs almost for free, I got some thin sheet steel, welding wire
for my mates MIG and spray underseal, plus flap disks, wire brushes and
cutting discs
Treated the same area on my zs a couple of years ago. Caught it early, so got away with elbow grease, rust eater and rubberised black underseal. I hope you get it sorted.
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