Many users ask: How do I use body filler (or Bondo as it's product name).

Using body filler for the first time can be quite an intimidating feat. Without it, molding your screen to your dash can be quite difficult.

This is a basic guide to using body filler for the first few times. It is by no means all inclusive, but it should help get you started and help to ease some of the tension you may have

Most body filler is essentially a fiberglass (polyester) resin. Many people wonder about the ability of todays body filler to adhere to various metals and plastics. With current technology, adhesion is just as good, if not better, than many of the epoxy adhesives out on the market.

One of the first steps that should be taken every time body filler is to be primered is to completely prep the surface that you are applying the body filler to. This includes:

- cleaned surface (many times, an alcohol pad will do the trick)

- sanded (a very rough surface makes for better adhesion) with 40 grit sand paper

- dry (moisture will prevent a bond from forming between the body filler and the material you are trying to bond to)

- nonprimered (many times primers contain chemicals that will prevent a bond from occurring between the body filler and the primer its self)

About Body Filler
There are two parts to body filler.

The filler its self:

And the catalyst (MEK):

The product you get from mixing these two is the result of a chemical reaction. Heat is a byproduct. The hardener (MEK peroxide) is mixed in with the resin and the chemical reaction begins.

Important Information
The surface of the area you are working on needs to be at least 64 degrees Fahrenheit (17.8 degrees Celsius). If it is not at this temperature, you risk the mixture curing from the outermost surface area inwards. This can mean that moisture and various solvents can cause bubbling, adhesion problems, and downright frustration at a later time. This could be minutes, days or week later, but it will happen. The ideal temperature for using body filler is between 72 and 80 degrees Fahrenheit (22.2 to 26.7 degrees Celsius). We all know that ideals are hard to come by. If your surface is cooler than the minimum recommended surface temperature, you can heat your work piece to the desired temperature. Just remember, you can get too hot.

One solution that many people resort to is using more catalyst than normal. While a minimal amount more is ok, as with everything, you can use too much. If you mix it too hot, you will cause the body filler to become brittle. This will cause cracks, chips, and peeling, which all can lead to big headaches. Keeping a good cure time will allow the gases to escape and will give you time to work out air pockets while applying with your spreader.

The recommended amount of hardener is 1.5 to 3%. Great, but how am I supposed to apply that in the real world?

One tip I've heard for mixing is as follows:

On the pallet or board that you mix your Bondo or filler on, put the amount you are going to mix up in the form of a circle. With your spreader or mixer divide the circle in half, 50%. Divide the half in half, 25%, half again, 12.5%, half again, 6.25% half again, 3.125% (this is the maximum) half again, 1.5% (this is the minimum).

Between coats of filler, do not wipe with solvent (thinner), because solvent will be absorbed into the filler.

Roughen the area where you are going to be applying body filler and then be sure to use an air hose, or compressed air of some kind, to clear off all of the dust. Mix up some body filler and apply it, being sure not to make any one layer more than ¼” to 3/8”.

Mixing Body Filler
Here is an example of a pallet used to mix body filler on:

Many people turn to a glazing putty to fill in pin holes or other minor imperfections in their work. Just remember that if you use this over something that didn’t have any hardener in it, it will absorb any solvent or thinner into the uncatalyzed product, and not the catalyzed product. This will cause the uncatalyzed product to swell a bit from any solvent absorbed. It will then shrink, exposing any imperfections in the putty finish.

To prevent this from happening, it is recommended that you use a two part system the whole way through.

One very important thing to note is that if you have ANYTHING that moves, even in the slightest, it will cause a problem in the future, usually, in the very near future. Yes, body fillers do have the ability to flex some, however, a crack will end up showing up later.

If you end up with a few cracks, there are a few options for you. First, you will need to remove ALL of the old body filler before going another step further. You will need to start over from scratch as mentioned above.

Some notes for mixing your body filler:

Do not use cardboard for mixing your filler. Cardboard could have been anywhere and could have any number of things absorbed in it, it can also absorb some of the solvent in the resin. The number one reason for not using cardboard for mixing on is that the styrene will cause any chemical that’s already on the cardboard to come out and be mixed into your body filler. There are some products available for this kind of thing, Clean Sheets is one that comes to mind.

Regarding what grit sand paper to use, I found this handy guide:

-16 to 36-grit for grinder discs for paint removal and bare metal Bondo prep

-24 to 36-grit for roughing out the filler with an air file sander or hand board

-60 to 80-grit for smoothing out the rough scratches from the previous operation and shaping finer contours

-80 to 180-grit for further smoothing in preparation for high-build catalyzed primers (with hardener), or 240 to 320-grit for lacquer-based (non-catalyzed) primers

-320 to 600-grit for final sanding prep before paint, following the manufacturers guidelines on their product information sheet

-600 to 2000-grit for sanding clear coats for re-coating clear, or for removing minor surface imperfections prior to polishing and buffing

Pictures of the process
You’ll want to buy a bunch of these. The nitrile gloves tend to be a better glove.

Getting the body filler out of the jug.

Body filler waiting for catalyst.

Body filler with catalyst ready to be mixed.