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From Wikipedia...

Bondo is a 2 part putty used to fix dents in cars, created by Bondo Corporation. While the term "Bondo" is a brand name for this company's product, it is commonly used as a genericized trademark to refer to all auto-repair putties or so-called plastic body fillers. Bondo is a two-part epoxy that when mixed turns into a putty which then sets and becomes rock-hard. The user can apply the mixed Bondo to the dented panel, sand it to the proper shape, and prime and paint it like the metal around it. It may also be used to Modify the look of PC cases, as some people do to make it look cool, using the bondo as a sort of clay to make items to paste on the case.

When building a home arcade cabinet, Bondo is commonly used to repair errant saw or router cuts, holes left when countersinking screws, and other applications. Wood filler or putty can be used as well, but sometimes wood filler expands over time, leaving a visible mark.

How to Use Bondo

Bondo is great stuff. It is easy to work with while it is soft, and can be sanded very smooth once it is hard. Unlike wood filler, Bondo will not shrink or expand which is nice as you know what you are going to get when you apply it. It will come either come in a pint or gallon sized can. Usually there is a rather large cap on top. Inside the cap are 2 things. A scoop and a tube of toothpaste looking stuff. Do NOT use it as toothpaste. Duh!
The cap is said to be where your supposed to mix it. The problem is that once it is mixed and becomes hard, you have to break it out in order to use the cap again. I find that the best thing to mix bondo on is a slick flat surface, like a coffee can lid or a piece of hard board. The scoop, I usually toss because who wants to clean a bondo scoop every time you need bondo. I usually use a paint stir stick to scoop the bondo out of the can. You can then toss it as the stick was free.

To mix the the 2 elements together I use a credit card. (This is a great use for all the "free" cards everyone sends you). With a credit card you can smear the smear and mix together, and you can also use it to scrape your surface to get all the bondo together into a pile again. A credit card is also thin enough to be flexible but strong enough to push the bondo around without breaking.

Another great tool to mix and apply bondo is a metal putty knife (The type used for drywall paste). But make sure that you get a flexible one not a stiffy.

The Bondo instructions say to use a golf ball sized scoop to about 1 inch of the paste. I do not know about you, but I have never seen bondo come out of the can in a way that even closely resembles a golf ball. So use your eye. I usually will scoop about 2. in square blob that is about 1/2 in. thick.

Ok here is the run down...

  • You have used your router in a way that has ruined your flawless cabinet. (oh my)
  • Clean off the area thoroughly. Get all the saw dust and what ever else away from the area.
  • Get your coffee can lid, bondo, and toothpaste ready FIRST. (you do not want to be fumbling around trying to get organized)
  • Get your blob of bondo (see size above)
  • Put a 1 inch line of the hardener resin on top of the bondo
  • Mix VERY well. This is important to the hardness.

Time is now ticking! With bondo, time is NOT your friend. It does not dry like glue, or wood putty. It is changing states through a chemical reaction.

  • Using your credit card, grab a small portion of the bondo and squish it into the gaping wound in the side of your once beautiful cabinet.
  • Keep filling in the area until it is full. The bondo may start to move as a whole. try to keep it put as you spread it.
  • Once the gap is filled also lightly swipe some around the edges of the gap onto the surrounding wood. Make this as thin as you can while making a constant surface.
  • It may be necessary to build up the damaged area instead of trying to fill the whole thing in one shot. If it is too deep you may have to fill in a little bit, let it harden, then mix up some more and top the original area. The cool thing is that you will not have to wait too long for the second application as the surface of the first should get just hard enough to re coat within about 7 or 8 minutes.
  • If the Bondo starts to get hard it will become difficult to work with. There is a point at which you have to stop applying it as you will make a mess trying to make work for you. It is better just to mix up more once this batch becomes a little harder.
  • As it becomes semi hard, this is a good time to clean up any stray Bondo drips. Again, use your credit card to scrape off the little bondo drops. If the bondo is hard enough, these should pop right off.
  • Once you are happy with the Bondo application, you need to wait for about a 1/2 hour. (Assuming you are of legal age) Margaritas are a nice time filler, but then again, this is probably how you got yourself into this mess to start out with. Isn't it? Myup.
  • When time is getting near, lightly do a finger test to check the consistency. If it feels hard and warm or is tacky to the touch, it is still probably curing deeper within the bondo. Wait a little longer. If you have waited more than 45 minutes and it is still sticky, you probably did not add enough hardener, or you did not mix it thoroughly enough.
  • If it seems cooler and totally hard, then it is time to sand.

Sanding Bondo is easy but messy. Wear a face mask as you DO NOT want to breath in Bondo dust. I use an electric sander with 60 - 100 grit to get started. This will take the bondo down pretty quick. When you see that you are getting close to your surface switch to like a 220 grit. This will take a little longer, but, the result will be a perfectly smooth surface to your original surface. Keep feeling around the edges. You do not want to feel any raised or sunken areas. Once smooth, clean up the bondo dust and call it done.

Another suggestion (related to bondo dust) is clamp a vacuum hose near where you are sanding. Most modern vacumes are strong enough to pull the dust from several inches. what works, right?!?
Update... Recently I have started removing the little dust catcher bag off the back side of my sander. I then put the hose of my vacuum right on the output of the sander. This does a good job of sucking all the stuff that would normally get caught in the sander bag, plus a lot of stuff that would normally get sent into the air otherwise. I still like to wear the face mask because it makes me look like a doctor, oh, and it keeps the dust out of my lungs.

See Also