Proper Use and Preparation of Bondo Body Filler

Curing Bondo Body Filler

Body filler is generally cured / dried within 15 minutes. Properly mixed, body filler will thicken in 10-15 minutes, and be fully dry [cured] in 30. At times however Bondo / body filler does not dry adequately. If the Bondo hasn't dried there are several reasons. The most common is simply that you did not use the correct amount of hardener. If your bondo hardens too quickly, you probably used too much hardener. You should only mix what you can use in 10 minutes or less.

 

The second most common reason Bondo doesn't cure correctly is that it's too cold. A hair dryer would be helpful, a heat and strip gun would be better. Just keep it moving as it could easily scorch the filler and car finish.

If the Bondo got wet, I suggest you start from square one. Even though it may have dried out the metal behind it, subjected to more moisture may have been oxidized and compromised.

The following article from Napa is helpful for more extensive information on working with Bondo

BONDO 101 – BASICS FOR WORKING WITH PLASTIC BODY FILLER

There is not a more maligned automotive product than “bondo”. Plastic body filler is most often referred to by the brand name “Bondo” (made by 3M), and its bad reputation comes from decades of poor application and misuse. Every high-end custom car or restoration has body filler on it. Using it on your daily driver or project is not a bad thing, as long as you use it the right way.

Like most plastic body fillers, Bondo is a lightweight polyester-based plastic body filler that was originally developed as an alternative to lead body solder. Old school body shop techs used to melt strips of lead with a torch to fill body imperfections and joints. . .

There is no minimum thickness for Bondo, but there is absolutely a maximum. You do not want to apply body filler to anything that is thicker than about a 1/4”. That does not mean it can’t be done, but the repair will not last as long. Eventually the filler will shrink and possibly crack, or even pop off! The general rule of thumb is that if the dent it deeper than 1/8” you need to do metal work first. Remember, your nice paint job depends on a solid foundation. Simply troweling on the filler may look like the easy button, but that slab of filler may decide to fall off some day (we’ve seen it happen). . .

Don’t use body filler on full gas tanks, or anywhere that gets hot. Bondo is resistant to fuel, but only after it is cured. If you are smoothing out a gas tank, that is great, it you are trying to fix a rusted out gas tank, this is not going to do a good job. Don’t confuse Bondo with JB Weld, the two are not the same. ..

PAY ATTENTION TO THE MIXING RATIO

The mix is crucial. Most of this comes from experience, but you can get close to the optimum mix with this tip- use 1:1 ratio of inch diameter of filler to inch-length of hardener. A 4-inch diameter of raw filler needs a 4-inch ribbon of hardener at room temperature. If it is hotter (above 85 degrees), use slightly less, if is colder (below 65 degrees), use slightly more. If your filler hardens too fast, you used too much, if it takes too long, you didn’t use enough. You only want to mix what you can use in about 10 minutes. Properly mixed, body filler will begin to thicken in 10-15 minutes, and be fully cured in 30. - Full Article From NAPA Knowhow Blog By J. Bryant