Missing underseal?

[Mikeso51]

I was working under back of the car (Blue GT) yesterday fitting some mudflaps and I noticed that there was no underseal, or indeed any protective coating, under the spare wheel well. This seemed a bit odd since the rest of the underside seems to be well protected with underseal and plastic covers. Is this normal?

[RUM4MO]

Unfortunately yes, there should be a light coating of wax in these areas though, but eventually that will get worn off, I have Waxoyl and intend to sort that area out.

[SRGTD]

Yes, it’s normal. Underbody areas likely to be subjected to potential damage from stone chips or other road debris are pretty well protected by the application of a stone chip protection or the fitting of plastic covers (underbody floor trays and wheel arch liners). The spare wheel well is much less likely to be subjected to stone chip damage, so I’m assuming that’s why it doesn’t get the flexible stone chip protection applied to it. The spare wheel well will get wet from rain, road spray and road salt, but I’ve never had any issues with the spare wheel well corroding during my period of ownership of any of the VW’s I’ve owned - all kept between 4-5 years (the body shell is galvanised when the car is built). I do regularly use a high pressure hose on the underside of the car when I wash it during the winter months, to wash off any salt deposits.

The discussion thread at the link below from back in 2017 includes some photos of the underbody of a couple of GTI’s (the second one is mine) that you can compare to your car. You can see no stone chip protection on the spare wheel well;
viewtopic.php?f=56&t=69884

[wolfie]

I was working under back of the car (Blue GT) yesterday fitting some mudflaps and I noticed that there was no underseal, or indeed any protective coating, under the spare wheel well. This seemed a bit odd since the rest of the underside seems to be well protected with underseal and plastic covers. Is this normal?

Unfortunately that sounds correct. I think it does have a thin coating of some sort but not much. I don't know if it's down to cost cutting but half expect to start seeing the "we won't colour paint what you can't see" that Fiat, Citroen and other have been doing for years.

Bilt Hamber, Dynax UC is a good product for protection. It's clear and pretty thin.

[veteran]

Mikeso51,

I'm adding my penniworth.

Yup, what you found is normal, though I too wondered about it when I acquired my Polo three years ago and first ventured under the back underside of it. The wheel well is not only a source of background panel-drumming but, as someone else has pointed out, its outside surface is constantly subjected to chipping from road stones, so one would think that it'd be coated in much the same way as the rest of the car's underside. Not so. My theory is that the proximity of the exhaust pipe and back box (fire risk) is the reason why it's not been factory-coated. Mine had not even been painted over fully.

I ended up buying from Halfords an aerosol underseal with a similar colouration to the surrounding paint, and applying that. Being from an aerosol though, I couldn't achieve a typically thick coating against stones. So I left it as that, thinking that, yes, I could, if I wished, additionally paint on to it a truly thick black underseal, such as Finnigan's, but that would reinstate the fire risk.

[Mikeso51]

Many thanks to all those who replied so promptly. Mine is exactly like those cars in the photos in the link posted by SRGTD. As Veteran points out, as well as a possible area for corrosion, this uncoated area is probably a source of noise in the car. It certainly resonates nicely when I tap it. I have a part pot of Wurth flexible water-based underseal which I bought some years back to touch up the damaged underseal on a Porsche I owned at the time. It's similar in colour to the existing underseal and goes on very thickly, so I think I will give the bare area a coating.

[Bepis]

Just some personal experience. The only area with considerable rust on my 18 year old 9N is the underside of the spare wheel well so I guess they haven't put much protection on the spare wheel well for some time.