Applying Body Filler

Here's a technique for applying car body filler (bondo). Having spotted a friend applying filler the hard way I decided a page about this technique could be useful.

The trick is to use a steel rule to apply filler.

Stage 1: Filler

In the photo a 1m steel rule is positioned at an angle on a curved panel allowing it to bend to the shape of the panel and highlight the position of a dent. It has to be a steel rule as they bend nice and evenly. Aluminium ones can kink.

There is a gap between the steel rule and panel where the panel needs filling. I tend to pencil a line on the panel around the edge of the gap to show where I need to apply the filler.

Using ruler to find dents

The filler goes on in the normal way using a plastic scraper to plaster filler in the area I've marked out. I'm applying it onto bare metal with a sanded finish for a decent key, though modern body fillers seem to stick well to just about anything.

Filling on top of paint is problematic - paint sprayed on top will sink into the filler a different amount from the surrounding paint as it cures, and you end up with lines in the paint around the filler. With cellulose paint these lines can keep appearing a week or two after painting as the paint cures.

Filler applied

The steel rule comes back into play in smoothing the filler. Pulled fairly tightly against the panel the rule is dragged across the filler. Most of the filler in the photo is being removed leaving only the filler that needs to be there. This vastly reduces the need for sanding to shape and wastage of filler - the filler removed by the rule can be used elsewhere on the vehicle.

The technique works very well for short lengths of filler. For areas longer than 200mm the drag of the filler on the rule makes the rule bend around the filler. In this case using the rule a second time can remove more excess filler.

Smoothing filler using ruler

Here's the panel after filling. I've missed a dent on the top left of the photo, but other than that some light sanding and stopper would make the level fairly close to correct. I use 80 grit sandpaper - it clogs quickly, but can be used accurately and the scratches can be finished off reasonably well before paint.

Some more checking with the steel rule shows which areas are too high and which need more filler.

Filler smoothed

My dent covered slightly more of the panel than I first thought, so a couple more applications of filler were needed. You can just about make out the pencil lines where I've marked out the panel for more filling.

Pencil might not be the best thing to use for marking out - filler is unlikely to stick to pencil graphite. While the lines are small, and I've not noticed any problems in the past, it would probably be better to use a marker pen.

More filler

Up to this stage I've only used dry sanding. During application the filler has lots of pin holes and pockets which can trap water. Water trapped under the paint will tend to bubble up on a hot day, so it's a good thing to avoid.

My preference for sanding is 80 grit sandpaper by hand using the rule to check levels rather using a sanding block. With hand sanding you have more of a feel for the panel shape. Between each coat I'll use a brush to remove sanding dust from the pockets in the filler.

Final sanded filler

Stage 2: Stopper/ Knifing Putty

Filler doesn't end up with a smooth enough finish for paint. Apart from the 80 grit sandpaper scratches there will be pinholes and various other imperfections.

Knifing putty can be used for finishing small imperfections in filler, but it's name is deceptive. In the past I'd always applied it locally using a blunt knife (useful for small imperfections), but it can also be applied with the same type of plastic scraper as used for body filler.

It can be applied in as thin a layer as possible over the whole panel. Knifing putty is a cellulose based material - so it will sink. Avoid using it on deep scratches - a last bit of body filler is the answer for anything deep.

The little tube of stopper in the photo would do a whole car. It's available in tins, but I prefer tubes as they don't dry out between projects.

Stopper applied over whole panael

For the first time I'm wet sanding using 240 grit sandpaper.

Generally sanding blocks will scrape at the edges more than the middle. For this reason the block should be used at an angle to even out the effect of the edges and prevent scoring.

Filler absorbs water, so I'll leave it to dry for at least 24 hours before painting.

Stopper wet sanded

The panels on this side are now ready for paint. I've gone over the bare metal with 100 grit sandpaper (to remove the surface rust that appeared instantly on wet sanding).

The panels feel perfect to the touch, but they are still not good enough for a gloss paint finish. The next stage in filling is spray on filler primer (high build primer).

Shell prepared for paint

Filling a painted surface

It's common advice to strip a panel to bare metal prior to filling, but I don't generally find myself doing this. Modern fillers will stick to most paints with a sand paper keyed finish. The photo shows a dent I didn't spot earlier.

The main risk is paint sinkage and edging, especially when using cellulose paints. New paint will be absorbed into the filler and surrounding paint at different rates, and the dried paint will end up with an edge at the edge of the filler.

To minimise edging I'll prep the paint before filler with 240 grit sandpaper and finish the filler and surrounding paint with 400 grit before over-painting. This also reduces sandpaper scoring which looks really ugly when it shows up in finished paint.

Filling over a painted surface

Filling over welded repairs

It's very common for seam welded repairs to body panels to have small pinholes, especially if you use a stop start welding technique to reduce blowing through. After finishing (grinding) the weld, hold a light on one side of the welded panel and look from the other side for any pinholes. These should be welded and ground before filling otherwise water could get through from the reverse of the panel directly behind the filler and cause bubbling. I'll tend to seam seal the reverse of any welded repair as a added precaution against pinholes I might have missed.

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