The Panda (and probably the Ford Ka) is the same.. it's a handy little tray for keen gardeners to germinate seeds in..
There's a drain hole at each end but they become blocked with even regular grit, never mind pieces of compost. Luckily the metal seems to be fairly robust, so it won't rot immediately but it's a good point to clean it out every three months or so (or whenever you're in the wheel arch).
If you apply paint etc. in there, make sure the drain "holes" (actually just a small kink in one of the panels that doesn't lie flush against the facing panel) don't get blocked by the paint.. and that paint doesn't run out of the hole, or you'll get a paint run down the inner chassis rail. It would bother me to see it, when I'm down there changing the oil or whatever.
Lately (maybe they're on a marketing push) I've been getting adverts on my Facebook etc. for something called Lanoguard. It looks like they squeeze a sheep until all the oil comes out of its fur.. (?) and then they turn it (the oil, not the sheep) into a grease, similar to Waxoyl or BiltHamber cavity wax products.
It stinks of sheep apparently, until it dries.. so what's not to like?
... but the reviews about it seem positive, so I'm tempted to give it a go... The only downside is that I'll have to jet wash the underside of my car thoroughly otherwise I'd be waxing over dirt and grit, which doesn't seem "the best way to do it").
Anyways, it covers everything (sub-frames, body etc.) and dries clear, so the surface just looks "waxy". Their USP is that it can withstand a jet-washer, so it won't get washed off.. but as with all products, they recommend an annual top-up (though even once every 2 years etc. has to be better than nothing).
Ralf S.