STOP, do not start putting the car body filler (bondo) on your classic car just yet. Before applying the body filler, has the underlaying body work been completed in a manner that will minimize the use of the bondo work? No matter whether you are working on metal or fiberglass panels the prep work is key to your bondo job and the finished product. The foundation is crucial to achieving a good finish that is ready for paint. You do not build a house on a Jello foundation; the same is true here.
Body filler has long been a standby for repairing small car dents. By filling in small dents, creases, and holes in the surface of the auto’s body, car body fillers can provide a simple, inexpensive fix for minor cosmetic damage on side panels, bumpers, and more. Even some major dents can be treated with car dental fillers. Unfortunately, filling compounds such as Bondo also present some drawbacks. Fillers like Bondo can crack if they are mixed too thickly, harming rather than improving the look of the car. Additionally, there is a risk of discoloration and poor long-term wear when you use a dent repair filler like Bondo.
When your body components are ready the following is a simple technique for applying car body filler (bondo). The trick is to use a steel rule to apply car body filler. The general rule of thumb is that if the dent it deeper than 1/4”, you need to do metal work first. That does not mean it cannot be done, but the repair will not last as long. Eventually the filler will shrink and possibly crack or even fall off.
First Step: Body Filler
A 1” wide steel bar 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 must be a steel rule as they bend nice and evenly as you will find very few surfaces on classic car seem to be truly flat. Aluminum ones can kink.
There is a gap between the steel rule and panel where the panel needs filling. We tend to pencil a line on the panel around the edge of the gap to show where we need to apply the filler.
The filler goes on in the normal way using a plastic scraper to spread the filler in the area we have marked out. We are applying it onto bare metal with a sanded finish for a proper adhesion, though modern body fillers seem to stick well to just about anything.
Reminder that body filler is a 2-part material where you will need to add hardener in the filler before applying it to the car. Be care not to add too much hardener to the filler as it will dry too fast for you to apply properly. Alternatively, not adding enough hardener will result in the bondo not hardening at all, and even more of a mess to cleanup.
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.
The steel bar comes back into play in minor smoothing the filler and checking your work on the panel. Pulled tightly against the panel the rule is dragged across the filler area. 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 waste of filler.
The technique works very well for short lengths of filler. For areas longer than about 8” 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.
Here is the panel after filling. We do some light sanding to make the level close to correct. We 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 bar shows which areas are too high and which need more filler. The dent covered slightly more of the panel than we first thought, so a couple more applications of filler were needed. You can just about make out the pencil lines where we have 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 we have not noticed any problems in the past, it would probably be better to use a marker pen.
Up to this stage we have 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 is a good thing to avoid.
For those who have some time spent on body work our 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. For larger areas, an important reminder when sanding body filler is to use a block (wood or plastic) to avoid ridges in the body filler. If you sand with bare hands the sand across your palm and not in the direction of your fingers as that will cause high and low spots. Between each coat we will use a brush to remove sanding dust from the pockets in the filler.
You can also you your hands to feel the panel to assess any high or low spots that are not easily “seen.”
Second Step: Applying Glazing Putty
Filler does not end up with a smooth enough finish for paint. Apart from the 80 grit sandpaper scratches there will be pinholes and various other imperfections.
Glazing putty can be used for finishing small imperfections in filler, and it can also be applied with the same type of plastic scraper as used for body filler. In general, we use glazing putty in a tube, and place directly on the plastic scraper edge before smoothing it onto the car panel.
Simply force the glazing putty into the holes as you spread it over the affected area. This should leave just the holes filled and truly little filler on the surface. You can then hit the area lightly with a block and 180-320 sandpaper to make sure nothing is above the surface. It can be applied in as thin a layer as possible over the whole panel. 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 is available in tins, but we prefer tubes as they do not dry out between projects.
For the first time we are 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. Try sanding at crossing angles to avoid creating ridges or low spots.
Filler absorbs water, so we will leave it to dry for at least 24 hours before painting.
The panels on this side are now ready for paint. We have 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).
Filling a painted surface
It is common advice to strip a panel to bare metal prior to filling, but we have not generally found ourselves doing this. Modern fillers will stick to most paints with a sandpaper keyed finish. The photo shows a dent we did not 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 minimize edging we will 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 ugly when it shows up in finished paint.
Filling over welded repairs & Seam seal
It is quite 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. We will tend to seam seal the reverse of any welded repair as an added precaution against pinholes we might have missed.
This is a simple plan for the do-it-yourself mechanic on classic car body repair. It would be best to practice on an old door or panel before starting on your own vintage car, if possible, to refine your skills. The good news with body filler is that if you make a mistake it is relatively easy to just sand it all off and start again. The only critical component of this process is to keep your bondo work to less than ¼” in thickness, and preferably to about 1/8” thick.
This method can be used for simple spot repairs; but if you are going for that show car look then more extensive body filler work will be needed across the vast majority of the metalwork. For the DIY project this process will get you a driver quality look without the mess created by sanding of filler product. For those who are on a tight budget there is quite a bit of money to be saved by doing the sanding and prep work before shipping your car off to a body shop for the larger scale finish work and painting.
