Automotive plastic body filler, often times called "Bondo," is used quite frequently for automotive repairs and restorations. Before using Bondo/body filler, there are a few steps of prep that will ensure the body filler adhere's to the surface correctly.
Mixing Body Filler
Mixing is the most vital step when using body filler. Plastic body filler needs the filler and a hardener mixed at the perfect ratio. The mix is crucial: 50:1. Read the instructions for a clear understand. Its important to note that proper mixing will ensure a consistent, even body filler that will prevent having to redo or repair the area again in the future. If the body filler is not mixed properly, you're not going to get the finish you want.
1. Mix on a Spreader Mixing Board designed for the job. Do not mix on cardboard, cardboard will absorb the important resins from the body filler and hardener. Lay down your filler and then apply the correct amount of hardener.
2. Place the proper amount of Bondo filler and hardener: 50:1. First, place the filler on the mixing board and then place the hardener.
3. Mix with a plastic or metal Filler Spreader. Mix thoroughly until their is a uniform color, any differentiation of color indicates the mixture hasn't been mixed enough.
Note: When mixing, do not whip or stir. Doing so will incorporate air bubbles into the filler which will cause more pinholes that need to be fixed later. Instead use a slow and deliberate motion back and forth that wiped the mixture across itself until even. Also, consider purchasing a Spreader Mixing Kit.
For a clear explanation on mixing body filler:
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