Posted by Alan Bowes on January 16, 2000 at 12:24:47:
In Reply to: Sheet Metal Repair posted by Roland Jones on January 13, 2000 at 20:11:27:
Lead is a good choice (with some practice) but if you are only applying a thin layer (less than 1/4 inch), a polyester-base filler will work beautifully and it should be a permanent repair. Proper preparation of the metal surface is important so that the filler gets a good grip. Use a professional-grade two-part filler (one that comes with tube of hardener/catalyst) for the main repair, plus a separate extra-fine-grained two-part filler for the final scratch-filling coat of filler. Follow all instructions carefully, use the right tools, and make sure that the top coat filler is compatible with the main filler material. I've used polyester fillers for over 30 years, and I've NEVER had a single repair crack or come loose. I've used a number of different brands, and they all seem to work well, except that some are easier to file and sand than others and some have more consistent/predictable curing times. By the way, I would suggest NOT using any one-part putty, such as the lacquer-based scratch fillers. These are OK when used on compatible prepped surfaces and when they are used only to fill small scratches, but they have some ideosyncrasies that require some practice, whereas the fine-grained two-part topcoat fillers do a comparable job (in my opinion) and are more dependable and easier to work with. For most repairs, I like to use the "lightweight" fillers, which have a matrix of tiny glass bubbles. They're just as strong as the fillers whose matrix is some combination of solid polyester particles and clay, and besides being easier to sand, I think that they are a bit more flexible without sacrificing hardness. The metal-filled polyester putties are a little harder, but not necessarily more durable as a cosmetic filler, and they tend to be more difficult to work and they also tend to clog up fine wet-sandpaper more easily. For long-term durability, flexibility is important, since even lead filler can crack if the sheet metal is allowed to flop around, such as on an unsecured fender.
One thing in favor of the polyester fillers is that they should bond very securely to the overlying primer or primer-sealer, due to the microscopic texture of the surface. This is especially true of the glass-bubble-filled putty, since the sanded top layer has billions of jagged micro-fragments that grip the paint. AND use a correct respirator when sanding any of these fillers, not just the glass bubble type (and when heating or sanding lead solder).
If you use a lead solder as a filler, you should prep the lead to bond with the primer. Some primers may be compatible with clean lead surfaces, but I prefer to etch the surface with a suitable acid.
Lead is an excellent choice when you are trying to build up thicker layers on non-flexing parts of a vehicle, such as "Frenching" headlights or recessing taillights (does that date me a little?) and you need a strong edge.
Have fun,
Alan