Removing Underseal From The Underside Of The Chassis

I finally got round to starting (that's 'starting', not 'completing') a job I've been putting off for ages: scraping the underseal off the floor of the car. 

Underseal removed from the underside of the car

As part of the prep. for replacing the outer sills, I had tided the inner sills/ box sections - which meant scraping some underseal from the sill/ floor join area. Rather than just underseal those areas again, I felt I needed to go all the way and scrape the whole floor area, de-rust and apply paint. before re-applying underseal in one uniform coat over the whole area. (So, another of those small jobs that grows and grows.....).

Replacing the outer sills

Overall, the underseal didn't look that bad, but there were tell-tale signs of it lifting - mostly on joins and corners. When it lifts, it traps water and that can lead to rusting unseen beneath the underseal. I already had signs of rust along the edges of the reinforcing strips along the length of the sills.....

 .......and around the seatbelt mounting points. Also, on closer inspection, there were other signs of bubbling on the floor pan  and so I wanted to investigate for peace of mind.

 The car was properly supported on four proper axle stands and what had been putting me off was the hours of back breaking working lying on the floor with the car only about 50cm off the ground. The underseal is bitumen based and so quite rubbery. You have to put quite a lot of effort to get beneath the surface to lift it. Originally I crawled under the car on a padded dolly, but found that when I applied pressure to the scraper - the dolly moved backwards! That didn't work!

Entering Limbo Land

I started on the floor pan beneath the rear passenger area and attacked it with a gasket scraper that I had sharpened. Some technique was needed but I found that with the scraper at just the right angle and my elbow wedged on the ground, the underseal came off in long, satisfying,  leather-y curls. I had to frequently re-sharpen the scraper. Occasionally it's edge dug into the metal where the strengthening ribs were pressed. Not enough to make a hole, but enough to stop me in my tracks and waste my energies. Cleaning around those pressed ribs was fiddly and time-consuming.

Working around the 'ribs'

I moved on to the area beneath the front passenger area. That proved to be areal slog..... Whereas the underseal had previously come off in curls. THIS STUFF DID NOT WANT TO BUDGE. It was far harder and even with considerable effort, only came off very reluctantly in tiny, dry flakes. The flakes splinted off in shards that got in my eyes, mouth and nose. I don't know if this difference is due to proximity to the engine and engine heat?

Underseal from the back and underseal from the front.
Spot the difference?

Another complication in this area is the domed recess where the front exhaust silencer sits. being domed, I couldn't get my scraper in at the right angle. Even at the time of writing I have not removed the underseal form the dome ends. I'm thinking I will need to use a wire brush on a grinder or drill.

Front floor and silencer recess

I also scraped the underseal from the area under the tank. On the plus side, the underseal was softer and came off the bigger areas in curls again. On the down side, there are pressed ventilation holes in this area so some fiddly detail work was needed.

Under the petrol tank. Actually very happy with what I found!

Although hard work, I think it was a wise move to remove the underseal. I found the floor covered in 'liver spots' of rust. Nothing more serious than surface rust but the start of what could have been a significant problem. Looking at some of the strips of removed underseal, what I found was tiny pin-prick holes on the outside. Almost too small to see and certainly not obvious when hidden away under a car.

Pin prick holes in the underseal........

Those tiny holes tell a different story on the other side of the underseal and you can see where water and rust are starting to get in - the 'liver spots' I found. Given that my car has been off the road for 20 years, all this occurred before 2000. If the car had been used in that time and I'd not bothered to tackle this job, that rust would be much, much worse.

......And the rust forming beneath

Because of the placement of the axle stands, I still have to do the area beneath the rear arm mounts and the boot floor area.

Still to do....

At the front, I still have to do the area beneath the front suspension mounts.

Front 'tusks'

For all my efforts, this is only the start of the 'first pass'. There is a still a sticky patchwork of grey covering the metal. It has the texture of linseed putty. I need to scrape as much of that off as I can with a flat scraper, as I think trying to remove it with an abrasive pad at this stage will soon see the pad clogged up.

Still work to do

I realise that there has been water ingress between the strengthening strips and the floor pan. I think it would be very destructive to remove and replace the strengthening strips based on what I've found - which is a solid floor. My aim is ti neutralise and convert what rust there is, and then seal the edges to prevent any more moisture getting in. 

So what have I learnt so far?

1. My garage floor is very hard

2. I don't want to do this job again

3. The original underseal was applied to bare, unpainted metal

4. That, by and large, it had held up very, very well

5. That it's hard to remove

6. Underseal is only as strong as it's 'weakest link'. It doesn't matter that 90% is good, if 10% fails and allows the car to rot.