How-To Strip Car Underside To Bare Metal

(Words and Photos: Dave Smith, American Car Magazine, June 2015 issue)

Whether you’re welding a patch or stripping the whole underside for rebuild, getting back to bare metal is the best place to start. Underseal is wonderful stuff, unless it’s just been walloped on to cover up a crap repair – I’ve heard many mechanics refer to underseal as “floor-in-a-can” around MoT time – and starting from bare metal is the only way to be sure there’s enough quality metal there to begin with!

First off, health and safety. Sorry. I have the Fox Mustang shell up on a rotisserie, which is wonderful. If you’re doing the job the car up on stands you’re in for a fun time of getting covered with crud, but please remember you have at least a ton of car perched above your head and use good axle stands. Working with power tools means that bits of cack are going to get flung far and wide, so eye protection is vital, and, as some of these attachments mean you have to run your grinder without a guard on, so is a pair of really sturdy gloves. If you’re just removing rust and paint, a simple disposable dust mask is also worthwhile.

All the attachments used here are for a standard drill or a 4.5” (115mm) angle grinder. A mains powered drill is best as they tend to spin much faster than cordless ones. Most of these attachments require the drill to be spinning at full speed for a long time, and put a fair old side loading on the bearings behind the chuck, so you either want a top-quality drill or a cheap one that can be treated as a disposable. Really cheap drills are a waste of time – I bought one for around a tenner from Argos a couple of years ago, and had wrecked it within a day and a half. The shaft ran directly in the plastic casing; there were no bearings behind the chuck!

This is the victim. Right, let’s see what sort of attachments we have!

(1) First off we have the twisted knot wire brush in the grinder. Quality varies wildly. I have used Zip wheels before now and they’re wonderful. This is a cheapo unbranded wheel from an autojumble. It’s noisy, not comfortable to use, but boy is it effective. It cleans everything off that steel, rapidly, and gets into creases and scratches without too much difficulty.

The finish it leaves is a very coarse key, ideal for thick primers or protective paints. It doesn’t seem to build heat up too quickly, but it’s completely unforgiving – get anything softer than steel in the way of it and it’ll just plough into it. It tends to hack rust and paint off in chunks, so dust isn’t as much of a concern as shrapnel, but being a cheapo it had one major problem: alopecia. It only shed a few bristles, but at that speed, a flung wire can stab you through your overalls, no messing about. Get the wheel stuck into some thick underbody protectant and it’ll redistribute it across 20 feet, but if you get it into any waxy, oily underbody goo, it’ll smear it around and it’ll get stuck in the wire bristles, requiring you to clean everything, including the wheel, with petrol or similar solvent.

When I’d finished, I found another problem with cheapo wheels – they’re made from two metal discs, riveted together with the wire bristles in the middle and a nut on the top to attach to the grinder. In this case, the rivet was not quite tight enough, meaning that I had to employ some mole grips and a lot of swearing to get it back off the grinder.

(2) Next up we have the Clean & Strip disc in the drill. Clean & Strip is actually a 3M brand name, but this version was another cheapo from the autojumble. I’ve used the 3M version and it’s brilliant, but the cheapo versions aren’t too bad.

Some come bonded to a mandrel; others come with a bolt-together mandrel like this, which is rather clumsy and can get in the way sometimes. Once it’s going, though, it’s an absolute walk in the park. You barely need to apply any pressure; it just spits away the paint, rust and clag, leaving a beautifully clean, brushed-looking surface that’s ready for paint. In fact, sod the paint, just hand me the clearcoat. It doesn’t make sparks or build up heat as long as you keep it moving, but it does create quite a bit of dust. Get it in the underseal and it’ll just magically clear it away. There are two versions – the grey/black one like this one, or the purple/black one that’s rougher, tougher and lasts longer. That’s the only main problem – longevity. They’re great on smooth, flat panels, and will last a good while unless you’re really leaning on them, but running across anything like panel edges, rust holes or bolt heads will see it start to tear chunks off itself. The minor problem is the size of the disc. They’re about four inches in diameter to begin with, which seriously limits the corners and creases you can get into. Once they wear down a bit you can get into more places, but with a significant reduction in speed at the edge of the disc.

(3) You can get these Clean & Strip-type discs on a backing disc to go onto the grinder, and here they really come into their own. They share all the pros and cons of the drill-mounted version with the added pro that, with the increase in speed, they’re even more effective. The added con is that it’s even tougher to get them into tight spaces, but on large areas like this spare wheel well, it’s the dog’s dangly bits. Note also that it won’t fit inside the guard on the grinder.

(4) Next was the ‘tulip’ wire brush in the drill. These are an inch or so across at their widest point and are good for getting into tighter corners; however, they’re not very well suited to heavy-duty work. The bristles are quite long and thin, so they don’t work as hard, plus it will lose its shape very quickly if pressed. And you’ll have to press, because their small diameter means that overall speed is low.

These are for tight spots and light rust or paint removal only. Try to use them on a wider area and you’ll be there all damn day, and if you get waxy underseal in them, it’ll never come out.

(5) Then there’s the flap wheel, so named because it’s made up of dozens of little flaps of sandpaper glued around the edge of a disc for the grinder. There are various grades of flap wheel, as there are various grades of sandpaper, and this one is pretty vigorous. You don’t have to lean hard on it to get it to strip through all the rust and paint; the only problem is that it will then keep on going. It’s also pretty useless at gooey underseals. It’s good for smoothing down imperfections, weld seams and rough edges, but you have to give it your full attention. It will create sparks, so take care around the fuel tank, and if you stay in one place too long it will build up plenty of heat, blue the metal and start grinding through it. They do last a surprisingly long time, though. Definitely one for the heavy-duty jobs.

(6) Last comes the ‘rosebud’ wire brush. This is even smaller than the tulip brush, at about half an inch in diameter, and is really only suitable for the very tight corners. They’re most effective if you can get them side-on to the job – they’re great for cleaning ports in iron cylinder heads – but your drill chuck won’t thank you for that. Small diameter means that overall speed is low, so they’re only useful for taking care of the very edges that the other accessories above can’t get to.

They’re also good for cleaning dimples such as spot-welds, but get underseal on them and you’re looking at soaking them in solvent. They don’t create too much heat, but every job you use them for will become tedious very quickly. One advantage is that, unless you bash them out of shape good and proper, they seem to last forever.

(7) Obviously there are loads of other drill- or grinder-mounted accessories suitable for cleaning up steel. There are also plenty that really aren’t. Some drill mounted wire brushes have brass bristles (while many are steel with brass-coloured bristles!), which are wonderful on softer metals or where the possibility of sparks is an issue. If you use them on anything heavy-duty such as a floorpan, though, they’ll be ruined in very short order and you’ll be picking wires out of your jumper for hours. Worse still is if the bristles find their way into an engine, or axle… There’s also the wheel made of strands of red fibreglass-type material. This is for very light-duty work only, and takes its sweet time to work through even a light coat of paint. It’s very gentle, if that’s what you need, but give it some real welly and it will wear out faster than whatever it is you’re trying to remove.

The results were pretty amazing. All the metal came up beautifully, with no hidden horrors, even in the seams. This is amazing given that Ford’s idea of underbody protection on this car was risible at best, and non-existent everywhere else. The wheelarches, though, have been coated with a really tough protectant that’s a hell of a job to shift – like trying to scrape chewing gum off a carpet – which I can only assume was added by a previous owner.

(8) Once the metal is bare, you really need to get at least a coat of primer on it as soon as you possibly can, ideally within the hour – rust never sleeps.

I’ve learned a couple of things from this experience. First, this job is several hundred per cent easier with the car on a spit. If I’d been lying on my back with all this crap dropping in my face, it would have been a real test of my patience. Secondly, whilst the unbranded, made-in-China stuff got the job done, paying the extra for the proper gear is money well spent. And thirdly, even accounting for stopping to take photos occasionally, cleaning this section of the boot floor – probably six square feet – took me bloody hours, and there are still plenty of nooks and crannies that you simply can’t get into with the power tools. Taking a bare shell to a blaster or acid dip might be a ball-ache in the short term, but if time is in shorter supply than money, it’s definitely the way to go.

Related Products

– 115m Angle Grinder

– Stainless Steel Wire Wheel

– Twist Knot Wire Brushes for: Single Hand Grinder, Angle Grinder, High Speed Drill

– Drill Strip Disc

– Shaft Mounted Strip Disc

– Angle Grinder Strip Disc

– Flap Disc: 115mm (40 Grit, 80 Grit, 120 Grit or Set of 3); 178mm (40 Grit, 60 Grit, 120 Grit or Set of 3)

– Nylon Abrasive Cup Wheel (for softer metals) – Fine or Coase

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