How to Mix Bondo

Bondo is one of the cornerstones of auto repair. But for all of Bondo's merits, its application can be tricky: when mixed, Bondo hardens extraordinarily quickly, such that mixing the entire can and hardener at once will easily waste the whole can. So when mixing Bondo, it's important that you do it correctly.

Things You'll Need

• Work gloves
• Bondo
• Catalyst
• Paint sticks (clean)
• Mixing surface (plywood or plastic lid)

Instructions

1. Open up your can of Bondo. The putty, if previously left unattended, may have separated into a top-lying layer of liquid, so stir the putty thoroughly.

2. Determine how much Bondo you need. Your can of Bondo comes with tube of catalyst; the whole tube will harden the entire can. So when mixing smaller batches, you will have to maintain that proportion. For example, you should use half a tube of hardener for half a can of putty.

3. Mix a small test batch. With a clean stick, remove a golf-ball-sized dollop of putty, and place it onto a clean surface--scrap plywood or a spare plastic lid will do quite well. The plywood will later go into the trash, but smooth surfaces can later be scraped clean with a putty knife.

4. Add the hardener in proportion. A golf-ball-sized section of putty should require about a centimeter string of hardener, squeezed straight from the tube. Make sure to use a separate, clean stick to stir the mixture, and keep that stick away from your unused Bondo. If you don't, traces of hardener on the stick may render your Bondo rock-solid while you're not looking.

5. Monitor your test batch to get a sense of how quickly the putty hardens; this is your working time. You will also want to get an idea of the eventual color of your mixed Bondo. While most auto repair jobs require a neutral pink, your ideal color will vary by the job or material.

6. Adjust accordingly. Only mix as much at a time as you can expect to use. If you need more time to apply the Bondo, you can try to add slightly less hardener than recommended. If you add too little, however, then your Bondo won't harden enough to complete the repair.

Tips & Warnings

• Bondo for boats comes in the form of a white putty, plus a liquid catalyst that gets added by the drop.

• Mixing bondo generates an exothermic reaction, so it can get quite warm. Exercise appropriate caution.