Surface rust on underside of sills GS300

[JeffL]

Just had my car MOT'd by the independent garage I've used for the last 25 years or so and it passed with no advisories (GS300 2007 with 97,000 miles on the clock).  I had a look at the underneath while it was up on the ramp and asked the guy if all looked ok and he said all okay but there is some surface rust on the sills (this is looking at the sills from underneath the car). I asked what were the implications of that and he said it's unknown how long it will be before it's a real problem, they can look ok for a long time and then all of a sudden can start to disintegrate. I asked what can be done and he said they could underseal and I asked for a ballpark price and he said probably about £120 using such and such or about £220 using something else. Prices + VAT and I can't remember the names of the two products he mentioned. The dearer job is a better product and requires two coats and would look better. I wasn't very quick off the mark and I'm not sure if he's talking about underseal of the whole car or just the sills. Probably just the sills do you think? It was Saturday morning and the regular office staff weren't in so I'll speak to them next week, but before I do that I thought I'd ask on here for any advice and guidance on how to proceed. Thanks in advance.

[Malc1]

There’s a simplistic view that undersealing an older car just seals in the dross that’s causing rust ……. no matter how much prep is done …… and it seethes away creating risk and rust and hiding it all 

NOW many moons ago I was chatting at a classic car show to a guy with an immaculate Austin Cambridge that had an unresolvable engine oil weep, from new, that spread across the underside giving best rust protection maybe  

Old age and rust kills cars …… eventually  Your car at 17 years young is in its prime of course and I’m sure you’ll find a good way forward 

Good luck 

Malc 

 

[steve2006]

If it is just surface rust then removal of the rust and rust prevention treatment should suffice.

For steel to rust the primary ingredients are moisture H2O and Oxygen O2 which combines to form FE2O3 Rust. Anti rust treatments if applied correctly inhibit both moisture and oxygen reaching the steel thus preventing the rust from forming.

The problem then is any gaps or cracks in the treatment allows the rusting to recommence except now it creeps under the treatment unseen so requires regular monitoring and remedial action.

The biggest issue is structural rust in sills is when it eats through the metal from the inside out due to the box section design, this is no longer surface rust as a poke with a screwdriver or an official MOT hammer will soon reveal

 

[JeffL]

Thanks for the replies Malc1 and steve2006. I getting the vibe from you two that it's probably best left alone but inspected maybe once a year? Correct me if I'm wrong. 🙂

[Malc1]

👍

Malc 

[Linas.P]

Sounds similar issue to what I have, mine was damaged around lifting points and surface rust. My solution was simply go over the trouble areas with wire-wheel and sprayed it with black Rust-Oleum paint which is both rust converting and good for protection.

Is that complete solution? Obviously not, but for time being it solved the issue cosmetically and at least slows down the car demise... it being 18 years old car, it is quite good going considering there is no rust apart of some surface level. And if that buys me 5 years before more robust solution will need to be used... this is plenty. 

[steve2006]

If you don’t mind a bit of DIY Jeff and have access to a rotary wire brush and drill it won’t hurt to clean it up and  apply some of the stuff mentioned by Linas. As he suggests it will hopefully keep that structural and possibly terminal  MOT failure off the sheet for a few more years.