Body Filler and Lead: One Man's Opinions
What is the best filler to use on my vintage car? I see this question many times over on many different sites, so I thought I would add my two cents in as a professional restorer.
First of all, let me explain that body filler, be it made of lead, aluminized reinforced polyester (i.e. All Metal or Metal-2-Metal), fiberglass reinforced polyester (i.e. Kitty Hair or Glass Lite), polyester filler (i.e. Bondo, Z-Grip, or Rage), or a glaze (i.e. Metal Glaze or Ever-Glaze) are all designed to do the same thing; smooth out small imperfections in sheet metal work. Each of them has there own strengths and weaknesses, which I will get into in a moment.
None of them are intended to fill large dents, more than a quarter inch deep. All of them, except glaze, are designed to go on top of bare metal. As well, some restorers apply them over epoxy primer with good results. Excepting glaze, they are not designed to be used on top of paint or most primers.
As in most things related to automobiles, the quality of the underlying work defines the quality of the final repair. Most horror stories regarding filler (i.e. Bondo) come the misuse of the product. You cannot use it over rust, existing bodywork, or apply it too thick and have a lasting repair. When used as intended, all fillers will last the life of the car.
In order to have a long lasting repair, they must be applied according to directions (I know, I know, but seriously, read the directions) and ALL rust must be removed. Any of these products used over rust will only buy you a few weeks or months and then the rust will return.
The manufacturers recommend no more than a quarter inch of applied filler. So, the sheet metal should be as straight as practical. What does practical mean? Well, if someone is doing a $2,500 complete paint job, it means doing as little sheet metal work as possible and slathering the car with filler. If someone is doing a $5,000 complete paint job, practical means all metal is finished to within a quarter inch overall. This is within the published limits of the polyester based fillers. If someone is doing a $10,000 paint job, it means getting the panels as straight as possible and using less than a 1/16 of inch of filler in small areas. While, I donít do it, I have seen shops use a reinforced filler to build up half-inch areas over a 2 foot x 2 foot section and have the repair last for several years. The manufacturers, and I, are not recommending it, but I have seen it work.
There are several classes of filler: lead, polyester, easy-sanding, reinforced, and glaze.
Lead is the original filler. It is a non-ferrous metal (i.e. wonít rust and a magnet will not stick to it) that melts at a low temperature and relatively (compared to other metals) easy to work. Itís advantages are: when the paint is removed from a car, one cannot tell whether the repair was done at the factory, before 1970, or by someone who used original materials. For some, it comes with bragging rights, "my car has no Bondo, only lead." The disadvantages of lead are: it work hardens as the car vibrates and becomes brittle over time; it is less flexible and has less adhesion than modern polyester fillers; it is more expensive; it is more difficult to work than modern fillers; and, the fumes it puts off while being applied are toxic. Personally, I see no reason to use lead, other than someone wants to be original and recognizes that the repair will be more expensive and less durable than one done with modern materials. Unless you are painting your car in lacquer for originality sake you should really not be using lead.
Regular polyester filler (i.e. Bondo, 3M Lightweight Body Filler, etc.) is made of polyester and a bonding agent. It is light, easily malleable, flexible, and has a low tensile strength. The repairs will typically have pinholes and need to be glazed before finishing. When applied in a thin layer, it is extremely durable and strong. However, it gets its strength from the underlying sheet metal.
You can think about it this way, it is very hard to poke your finger through plastic wrap once it has been vacuum-applied over a product you buy. But, it is very easy to poke a hole in a roll of plastic wrap. The strength comes from the substrate.
Easy sanding fillers (i.e. Evercoat Rage Extreme, Z-Grip, etc) have hattonite or talc added to them to make them easier to sand and have fewer to no pinholes. Trust me, get this type of filler, it is much less work, even if it is more expensive.
Reinforced filler is a polyester-filler that has some sort of strengthening compound added to it, generally aluminum beads or fiberglass. All Metal and Kitty Hair are examples. This filler has a much greater tensile strength and is suitable to use in places where lead would have been used by the factory. There is a bit of a religious war as to whether fiberglass or aluminum beads are a better, but both are very strong and flexible. Neither product will leave a finished surface and will need to be skim coated by another filler and or glaze.
Glaze comes in a tube and is designed to fill very minor imperfections. Glaze unlike other fillers, can be used on top of existing paint and primer, so it can be used to fill small surface imperfections (stone chips, etc,) before a car is painted. It should not be used in large areas or as the basis for a repair.
The way I approach the work is as follows. I do not use an epoxy base coat. I repair and work the metal to within 1/32 of an inch overall, with the occasional 1/16 inch problem area. I apply All Metal (aluminized reinforced filler) in the problem areas and for covering weld seams and factory panel issues. I may or may not use some All Metal to slightly change the shape of panel openings (trunk area, door openings, etc). This is dictated by how perfect the car is to be. I personally prefer to not fix factory gap issues, but to each their own. After I finish with the All Metal, I will use some Z-Grip (easy sanding filler with hattonite) in areas with small undulations that I believe will not block out during sanding. I then prime the exterior surfaces of the car with a high-build polyester primer (I like Sherwin Williams primer). This is essentially like spraying 30 mils of polyester filler on the entire surface of the car. I block the living daylights out of the car, and hope I donít need a second application. I will then use Metal Glaze (glazing putty) on any small areas as necessary and then seal and paint.
I hope this is helpful for those trying to navigate the world of fillers. This is just my experience, I am sure others will chime in.
1974 Alfa Spider Daily Driver
1965 Jaguar E-Type
1977 Ferrari 308 GTB
1965 Plymouth Sport Fury with a 426 wedge
Last edited by Vantaaj; 12-19-2005 at 05:21 PM.