How to Mix and Spread Body Filler
Virtually everyone knows about "Bondo." That's the brand name - and generic name, for that matter - of plastic body filler. It has a reputation for being an inferior way of smoothing dents and patching holes, but in fact the material itself is very stable and effective when used correctly.
That's the problem, however. Plastic body fillers are often not used correctly. Simply speaking, filler should never be used in any automotive situation where it isn't backed up by solid metal. Further, no air should be able to get to the back surface of the filler. The plastic is hygroscopic, meaning it will absorb water, and therefore will allow rust to eat away metal that isn't protected. Properly used, plastic body filler is wonderful stuff but if the underlaying material isn't done correctly the "rust demons" won't be far behind.
We're going to show you how to mix and spread the material, so roll up your sleeves and let's start. First, bear in mind that body filler is a two-part chemical mixture that, mixed improperly, will not harden and cure. Therefore, the first thing to remember is to mix the right proportions of the filler and curative agent and to keep the two from accidentally mixing with each other.
Okay? Now let's mix some. If you're not outside in the open air, put on a respirator. The fumes aren't unbearable, but they are potentially dangerous without adequate ventilation so make sure you are protected.
You need a solid platform on which to mix the filler. We often use stiff cardboard pieces about 6x6 inches, but always bear in mind some of the hardener will get absorbed by the paper surface. Special plastic sheets are sold for this purpose but they can get hard to clean after some use and we prefer the "throwaway" versions. The ideal mixing tool is the popsickle stick, or some equivalent. Don't use screwdrivers or other tools, because you can introduce oil or grease into the mixture and won't be able to properly clean the blades.
Now for the mixing: pour or scoop out about a golf ball sized wad of the filler and then close the can. Next, squeeze a ribbon of hardener across the width of the pool of filler. Using the stick, mix the two together quickly, but don't whip the mixture. "Fold" the mixture to minimize air bubbles and make sure the overall color is even. If you see streaks of filler or hardener, keep mixing. Adding too much hardener will result in rapid gelling of the mixture, making it hard to get coverage in time. Too little hardener will increase cure time, but there's a point where the filler won't ever harden and you won't know it until you try to sand. Stick with the proper proportions.
Now that the filler is ready to spread, things have to happen quickly. You have only about 3 minutes to spread it before it sets up to a gel, after which it won't adhere to anything.
Plastic spreaders are sold for body filler and these work very well. Also, we like to use drywall knives for fine spreading. They are stiffer and help spread the filler smoother and thinner, but are a little difficult to keep clean.
Spread the filler with an even pressure and moving smoothly over the area to be filled. Don't go back over the area more than once, as it's easy to pick up grit or dried filler and create scratches and air pockets. The next application will cover these problems, and you can rest assured there will be a next application.
Take a look at your work and adjust your technique for the best results. In 70-degree, dry temperatures the filler will set up and be ready to sand in 20-30 minutes, so a lot of filling/sanding/filling can be done in a day. Be patient and don't feel compelled to use up all the filler you've mixed. It will just end up as sanding dust.
NOTE:The photos presented in this article are for demonstration purposes only. Don't mix your filler on cardboard surfaces as the cardboard may contain chemicals that can contaminate the filler. For the purpose of our demonstration, we used a handy piece of scrap steel that had some surface rust on it. Filler should only be applied to clean, rust-free surfaces.