Expert: William Forster - 5/13/2006

Question
I have heard that auto body repairs using fillers will eventually break away or develope hairline cracks over a few years, is that a true statement?  Also it was stated that this is especially so when filling a crack of a half inch in width by about four inches long.  Again is it true the bond will show later? This is concerning a 1949 Dodge on the nose of the hood where it has separated.

Answer
Hello, and thank you for your question! The filler on your hood was either improperly applied, a low quality filler, or the hood has been flexed beyond the capability of body filler to take. When you say a crack, do you mean a crack in the metal, a crease in the steel, or maybe a 2 piece hood seam was filled in? Any of these things are improper application. SEAMS AND CRACKS MUST BE WELDED, CREASES AND DENTS MUST BE PULLED OUT! Most of my experience with filler cracks, over an open area, are the result of an impact in the filled area. Instead of a dent, you may get filler cracks. In your case, perhaps a person sat or leaned heavily on the hood. Once the paint is cracked, water can penetrate the filler and cause it to swell and crack. FILLER WILL SOP WATER LIKE A SPONGE! The paint cracks can be almost invisible, and mistaken for swirl marks or scratches from a dirty wash mit. Body filler is an excellent, long lived product when properly applied. It should be applied NO MORE than an  eigth inch deep. Dents should be pulled or pried back up to grade, then hammered and dollied as true as possible. ALL rust and paint must be ground away. Patches that are riveted on, or spot welded seams, will crack in less than a year. Unfortunately, filler will never be as good as, or as long lived, as the steel. But neither will the paint. I have a 1966 GTO that I painted in 1991 that the filler is still holding on, and one quarter had filler from a previous dent repair. It is all still nicely intact. Also, use a high quality filler. If you dont pay at least 25 bucks a gallon for it, its crap. USC and Evercoat have great, pro quality fillers. Hope this helps. If not, please clarify what you mean by filling a crack, and I'll get you a clearer answer.  Bill