BODY FILLER

Body filler has a poor and largely undeserved reputation for being the bodger's favourite. That's because it can indeed be used for covering up rust or poor- quality repairs. I still have an old 1970s edition of a book from a very respectable motoring organisation (who shall remain nameless to avoid any embarrassment) which explains how to use chicken wire to bridge holes in car sills and provide support for body filler! I have also in the past dug old newspapers, cereal boxes and biscuit tins out of cars that looked perfect at first glance, but only because somebody had spent longer sculpting body filler to perfection than it would have taken to carry out a proper repair in the first place.

However, if any corrosion has been cut out and new metal welded in properly, then a thin skim of filler on top to blend everything in is not only acceptable, but virtually essential. Indeed, for decades car manufacturers employed very skilled craftsmen to do just that in their factories, though they used lead rather than plastic body filler – we'll look at lead loading in a moment. Problems come when filler is applied over rust, which will then continue to fester underneath and eventually push the filler away to reveal an even bigger problem. It is also an issue when filler is applied thickly to restore a profile, or on a panel that is subject to flex. Sometimes both problems are present, for example when somebody has tried to blend an air scoop into a bonnet, and they invariably end up with the body filler cracking.

So the basic steps that must be taken before opening the tub of filler are to cut out any rust, carry out proper welded repairs, bring the surface as close to the required shape as possible without it standing proud at any point, and to clean the whole area back to bare metal. Only then should you apply a thin skim of filler. Some people apply a thick wodge and then sand it laboriously back down to level, but that does lead to a lot more work than if you apply a thin coat first, sand that back and then apply a second thin coat only where the first one has fallen short.

When you are sanding back the filler, always use a flat rubbing block or a sanding board because otherwise your fingers will rub grooves into the surface. Even better on less- complex shapes would be a dual-action orbital sander. The benefit of a DA sander is that the sanding disc's orbit is random, and so it creates the flattest surface.

Some body fillers are finer than others, but even the finest of them will probably leave the occasional pinhole in the surface after sanding back. You should use a stopper putty to fill these, as is shown in our Project Cortina feature this issue. Stopper (also called putty) is essentially an ultra-fine one- pack filler that is easily sanded back. Generally, on body filler 80 grit paper is used for the initial shaping, followed by something like a 320 grit for putty or for final shaping and preparation for paint.

With the paint removed either side of any repairs, welds can be covered with a thin skim of body filler. Note that this batch has been mixed on a metal plate, not on cardboard which can suck moisture out of the mix. Initial flatting back can be done with 80 grit paper in a sanding block to get it roughly to shape, but expect to add a second coat to fill any low spots. Final flatting back should be done with 360 grit paper, in a DA sander as pictured here or using a rubbing block, and extended into the surrounding paint.