How to Work With Bondo Body Filler For Auto Body Repair

Difficulty: Moderate

Instructions

Things You'll Need:

60 and 120 grit sandpaper
Square or rectangular shaped power sander
Hand held sanding blocks (Palm size and longer)
Plastic filler spreaders
Bondo, (long hair, short hair, premium or regular) Premium with no hair applies and dries smoother and requires less sanding afterwards versing long hair regular
Cream hardener in a tube, usually comes with the bondo
Pallet or mixing tray, smooth plastic surface
2-inch putty knife
WD-40 for cleaning
Bondo scraper molding tool (looks like a wood plane with cutting edges)
Flat edge ruler, level or straight stick
Glazing spot putty compound
Auto Primer Spray

Step 1

You want a wax free surface to apply the bondo to which is acquired by sanding the area to be filled and the surrounding areas. 120 grit paper usually works best in removing the surface waxes and paint, with your goal being to have a dull and dry surface for the bondo to stick to. If there is rust in the area, you will need to remove it.
Step 2

Ready to mix!

Get your mixing pallet, bondo, cream hardener, putty knife and the correct size plastic spreader together. Good idea to have a rag nearby too. Decide on the approx. amount of bondo you will need to fill the area in question, keeping in mind that you will literally only have a few minutes to work with the bondo once mixed so it is suggested you may want to mix just enough for one complete area at a time.
Step 3

With your putty knife take out the approx. amount of bondo you will need from the can and put it on the center of your mixing tray. You have time now to really decide on the correct amount you'll need, since the bondo itself will stay pliable until you mix in the hardener.
Step 4

The small tube of hardener is enough for the entire quart in the instance you purchased a quart, so you can see you will only need a small amount of hardener added to the bondo. Squeeze the hardener onto the bondo and use your putty knife to work and mix the hardener throughout the bondo. This should only take a minute or two. Scrape your blade clean on the side of the tray now and then. The premium hairless bondo will be thinner versing the long hair bondo which has a thicker texture.
Step 5

Bondo ready to spread from a smaller sized plastic spreader.

Pick up some bondo on the plastic spreader edge and apply. It may be a little runny at first so hold your mixing tray level and below the work area to catch any drips. Press inward some into any dent or creased areas, using smooth long single strokes covering the entire area from side to side or up and down, or both directions, whichever angle works best to obtain the smoothest finish, preventing any sags from drying by floating the spreader over the area to keep it flat. Once the bondo begins to get stiff and tacky, within minutes, you should stop spreading right away. Don't worry if it doesn't end up all that smooth, you will remove the excess once it dries hard, but you do want to build the height over the dent or crease so that it is flush or higher then the surrounding surfaces. However, if the dent or crease is very deep, you may have to come back later and build slowly with additional applications.
Step 6

Let it dry. This could take some hours, depending on how thick you applied it, how much hardener you used and what the temperature and humidity is. To test if is hard enough, try to dig your fingernail into a corner. If you can make a cut in it, it needs to dry longer.
Step 7

Bondo scraper molding tool and 60 grit paper on a square power sander.

Now use the scraper forming tool and scrape down the excess sharp edges or hairs moving the scraper up and down, side to side and in angular directions, keeping the scraper flush to the surface till all the hard edges are smoothed down as low as possible. If you used the premium hairless bondo you will probably not need scrape much, if at all, since it goes on and dries smoother and there will be no hairs to rake down.
Step 8

With your power hand sander, using 60 grit paper, sand over the entire area back and forth and from side to side to get the bondo surface as flush as possible to the surrounding surface and until the bondo edges feathers into the surrounding surface.
Step 9

Smooth down remaining edges and feather in with hand block sanders and 120 grit.

If you still have rough edges the power sander didn't remove, use the 60 grit on a hand block sander to smooth out and then move into the 120 grit paper, you want to sand down any remaining edges of the bondo and get them as smooth as possible feathering into the surrounding surfaces. The block sander should be long enough so that when you stroke the work area both ends of the sander also run onto the surrounding surfaces to obtain a smooth flush level surface.
Step 10

Check with straight edge to find low or high spots.

Now using your flat edge ruler, level or straight stick, check to see if the filled area is truly flush with the surrounding surfaces. Lay the edge across the area at different angels, looking for low or high spots. With the ruler on the car you can mark off with a pencil any areas that need more filler or possibly more sanding down if too high.
Step 11

Primer over bondo just before glazing with spot putty and block sanding.

After you have worked your bondo down as smooth as possible you want to spray prime the entire area which will allow you to see more clearly if any flaws remain. This picture is from the other side of the car's rear quarter panel, which needed the identical repair.
Step 12

Glazing spot putty compound for final smoothing and to fill in small pinholes or dips.

The final steps would be to hand block sand with 120 to 220 grit paper and use spot putty glazing compound to fill in any pinholes, smooth over your feather edging or to create a small amount of height if needed, no greater than 1/16 to 1/8 inch build at a time. If you have deeper pockets you may need to apply more bondo.
Step 13

Expect a little trial and error since this is like sculpting and I consider it an art, requiring a steady hand, and practice makes perfect. Clean up the mixing tray, putty knife and plastic applicators with some WD-40 or similar oil as soon as possible.

Comments

can-do said

on 12/23/2008 Hi, Maybe not under the brand name "Bondo", but I'm sure there are other "body fillers" available. Bondo is just a brand name that caught on for the filler system, just like Xerox did for making copies. There are other copy machines, if you get my point. Another label I use is called "Marsons Finishing Putty". It cost a little more than bondo but works excellent. The key is to get a (2) part product, meaning the filler in one container and the small tube, which is the hardener, to be added into the filler in small amounts.
Good Luck