Ever wonder what Bondo is, and how it works? Bondo is a complex mixture of tiny little shards of fiberglass or polyester, resin, and talcum powder. The fiberglass shards keep the compound as strong as OEM (original equipment manufacture) specification, the resin helps assure that the bond to the vehicle being repaired is as strong as it can be. The talcum powder keeps the material flowing smooth. Talcum unfortunately though, absorbs moisture and that is why the fillers absorb water. Which is why it’s so important that the bondo be completely dried before it’s exposed to the elements. The main solvent in the filler is styrene, which is supposed to vaporize as the mixture cures.
Body fillers are thermal-set plastics. That is, they cure with heat, created by the chemical reaction
between the filler and the catalyst, and become hard and stable, usually within a few minutes. Too much hardener will cause the cured material to become brittle and crack, and also create bubbles that trap the styrene gas. While too little delays a cure almost indefinitely, and usually needs ground out and reapplied. Temperature is very critical to exothermic (the fancy, scientific name for catalyst-induced heat. =) reactions. In fact, curing is dramatically slowed below a 64-degree temperature, to the point that it can easily take several days to become fully hardened. The best temperature to use filler is between 70-80 degrees, and that includes the metal that you are applying the filler too. If the metal is cold, the mixture will cure from the outside in, trapping moisture against the cold metal surface, creating rust pits. It can also trap unvaporized solvent, which could come back later to haunt you if you paint over it too soon, because you will end up getting fish eyes in the paint.
Mixing the filler on cardboard is not a great idea, since the paper itself will absorb some of the styrene solvent and upset the chemistry. Also, the styrene will release any trapped chemicals in the cardboard, so unless you know precisely where the material came from and how it was handled, use a clean sheet of plexy-glass or plastic or freezer paper on a wooden board.
The smoother the metal, the less perfect the adhesion will be, which is why the instructions always tell you to rough-sand the metal surface. Those scratches allow the Bondo to stick very well and for a very long time, assuming there is no rust layer. If there is a layer of rust, the bondo, or whatever kind of filler you use will separate and end up falling off. To help prevent this from happening, it is always best to completely grind or cut out and replace any rusted parts where the bondo is to be applied.
Since fillers stick best to metal, it makes sense that spreading them over metal with primer already applied probably will lead to separation later. That is true, but if the surface is still scratched and rough, the filler will tend to stick extremely well. If you use an etching primer you must make sure it is properly cured before using filler. Otherwise, the phosphoric acid vapors left over from the application of paint can slow down the cure of the filler resin.
Making sure the damaged area is clean of solvents of any kind, such as degreaser or tar remover to promote a good strong bond.
The recommended depth for perfect bonding is 1/8th inch deep, but it varies, if applied right it can be layered up to 1/4 inch thick and not crack or separate for years, yet if its not applied right, even 1/8th inch skim coat can crack after the first few months.
After the first rough coat is applied, let it dry for approximately thirty to forty minutes before sanding. Keep in mind that the first coat is only to shape the project back to its original form, and that you will need at the very least one more skim coat to make it perfect. Once the first coat has been sanded to the desired level, clean the area with an air powered blowgun with a psi rating of no more than 110. Wipe paper towels with 320 type paint reducer or a tack cloth, you should never use lacquer thinner over bondo because the talcum powder will absorb the harsh liquid ruining the paint. Unlike lacquer thinner, 320-type paint reducer vaporizes almost instantly making it safe to use on bondo or other surfaces that are ready for paint. Now that it is clean, you are ready to apply the final skim coat. Skim coats are only to help fade in the bondo to the paint and are never supposed to be thicker than 1/8 in. max. Okay sanding for the last time with 400 grit sand paper usually does the trick, any sand scratches left from 400 grit will be filled in with the first coat of primmer.
Time to paint; if you did it, right nobody will ever even know that there is bondo on the car. There is one way to test for bondo but the only way it works is if the bondo is thicker than it should be, try taking a magnet and running it over a car, (be careful not to scratch the paint) and if the magnet won’t stick, then there’s probably bondo there. If done right, bondo can save you hundreds of dollars because you won’t have to buy a brand new fender just to fix a dent you got at your local Wal-Mart.