Bondo - Instructions for Use

Bondo (body putty) is a very useful material when doing body work. It's cheap, easily accessible and fairly easy to work with. Bondo can take a little getting used to; hopefully this short guide will give you a few of the tips and tricks that we've picked up along the way to help you with your next body work job.

To start with, you're going to need a few cheap supplies in addition to the can of Bondo -

• A surface to mix the Bondo on.
• Some spatula-like devices to mix the goo.
• Some sandpaper, increasingly finer grit, 120 to 320 is fine.
• Optional equipment includes latex gloves to keep your hands clean, and a breathing mask (which isn't truly necessary, but it never hurts).

Bondo also makes a line of spatulas. They are the same "salmon" color that Bondo should be if mixed to the right proportions. You can usually find these spatulas right next to the Bondo at the store.

Coffee can lids make excellent disposable mixing surfaces. And when you're done, the Bondo dries and will peel right off.

1. First you need to prepare whatever surface the Bondo will end up touching. You will want to physically support the Bondo -- it needs to have a good healthy physical relationship with the surface. Bondo can span fairly large gaps and can take a good amount of flex, but if there's too much torsional pressure it will most likely crack. If you're trying to repair a crack or fix two surfaces together, it will save you hassle if you Epoxy or superglue them together before hand. If the Bondo is going to span small gaps (like a screw hole) I put masking tape on the back of the piece, so your panel doesn't turn into a play dough fun factory. If it's a larger span such as a fan hole, or a complex shape to be spanned (such as a curving bezel) I use wire mesh. It can be picked up at any hardware store or art supply store. A tight mesh is easily bendable and the Bondo will fill in between the cracks and work really well.
2. Next you'll want to sand every surface the Bondo will touch. It doesn't have to be heavy sanding, just enough to score the surface. This gives the Bondo that little bit extra to cling to. If you have stripped your panel down to bare steel with a paint stripper (or even if not), clean the surface with soap and water as the Bondo won't adhere very well to any grimy stuff that may be on the surface.
3. Now you will mix the Bondo. Here's where the special Bondo-brand spatulas will help you. It's important that you get the proportions right. Not enough hardener and you'll have a semi-hard or still wet Bondo goo; too much and it will harden while you blink. Don't mix up too much at one time either, as no one likes to see it harden while frantically smearing it over a panel.
4. First you will put down a fill layer. This layer's only purpose is to start to fill the holes. This might take any where from 1-3 layers depending all upon the hole shape. You will still be able to see the highlights where it the fill layer is below level.
5. Next is what I like to call the "level" layer as its goal is to try to be level with the surface. This layer shouldn't be nearly as thick as the fill layer (the Bondo is easier to control in thin layers).
6. Finally the "feather" layer. The purpose of this layer is to go over level (just slightly) and then to "feather" the edges. This is usually the thinnest layer and I think the hardest. You always want to go above level on the feather layer, as well as going the furthest with the feather on the edge.

Note: As soon as the Bondo starts getting a little tacky or lumpy and won't smooth over, stop spatuling. This means the Bondo has already begun to set and you'll do more harm than good by continuing to harass it.

7. Now onto the fun part, sanding! If you followed the filling instructions up to this point, sanding shouldn't be terribly difficult. With the layered method you can gradually buildup the general shape and have it pretty consistent before you even start to sand. Grab some 120 grit sand paper and a sanding block. Start sanding and try to get consistent coverage at the angle perpendicular to what it should be when finished. Make sure you never go below level or you're going to have to end up filling with some more Bondo.

Note: Sanding creates a lot of fine dust that loves your lungs, and your clean floor. A particle mask is recommended along with an outdoor sanding location if possible. A drop cloth or tarp can be used to collect the dust.

8. All this sanding would be fine and dandy, but even with a thin layer above level it's going to take all day to sand. Here's a little shortcut -

• A paint stripping wheel can usually be purchased where you got your Bondo, or at any home improvement store or. This little device will pay for itself many times over. It is simple to use and will fit your standard electric drill. The stripping wheel is made out of some sort of plastic based fibers impregnated with epoxy (sort of like glued Velcro). When spun they have enough force and friction to break up paint and Bondo, but when they hit metal, they tend to just leave very fine scratch marks instead of sanding (that's why they are paint stripping wheels instead of just metal stripping wheels.
• This is good news and bad news. The good news is that the paint stripping wheel will save a lot of time and can do what would otherwise take hours in a few minutes. The bad news is that they won't touch metal (not sure about plastic) so if you sand down to the metal while using it, it will just catch the Bondo and go below level.
• Always use a back and forth movement. Never stop, as the wheel will catch and dig in deep. Just go back and forth, trying for consistent coverage and trying to sand perpendicular to what should be the finished surface.
9. Now you can see the fabled "feather" starting to form. It extends just past the edge and is very thin. It should be thin enough to see the surface below it. When you get close it is advisable to switch back to hand sanding, as it's easier to get a more consistent surface.
10. Now check for areas below level or general unlevelness. You can use both touch and the underwater method. Just rinse your panel under a faucet for a few seconds, then pull it out and hold it level so just a thin film of water remains. With an overhead light source, check to see if the reflection comes out in the proper shape. If its wavy or if there are sharp edges, you either need to sand some more or fill.

Note: If you drop below level or miss a spot, or if some tough shape or detail is giving you problems -- have no fear, another solution.

There is a Bondo-brand spot putty that comes in a big tube, available where you bought the Bondo. It varies from the Bondo primarily in the fact that it needs no mixing and is a lot smoother -- like cheese sauce to Bondo's honey-like consistency. It can be applied in much the same way as the Bondo. You just plop a little on the edge of a spatula and spread it over in paper thin layers. Don't try spread it around too much as it cures even faster than the Bondo does. Just fill in any places below level or that need extra smoothing up. One of the main ingredients of spot putty is talc, so it sands very easily.

Wet sand with about 320-400 grit. Sand lightly to make sure you don't go below level again, then do the water test just to see if everything is to your liking.

11. If you're happy with what you've done, you're finished and ready to apply the primer.