Bondo ranks up there with all-time DIY hall of famers like WD40, duct tape, and vise grips. It’s versatile enough to use for kinds of household projects including repairing damaged wood, restoring old furniture, and patching holes in doorways and windows.
One of the characteristics of bondo that makes it so effective for household repairs is its drying speed. It dries significantly faster than other wood repair products, making the application of multiple coats in a short period of time possible.
That said, having an idea of how long bondo takes to dry is important whenever you’re planning out a project or repair.
When used according to the manufacturer’s recommendations, Bondo takes 10 to 15 minutes to dry enough to form a sandable surface. After 45 minutes it’s cured enough to be painted.
Of course, bondo drying times are influenced by a number of different factors, so the exact dry times will depend on things like temperature, the ratio of hardener to resin used, and how well it’s mixed together.
Factors Influencing Dry Times
Bondo is a two-part epoxy, which means it consists of a hardener and a resin. These two components are mixed together when you use the product, causing a heat-generating chemical reaction to occur which greatly accelerates the curing process.
Curing time depends on two main factors, temperature and the amount of hardener added to the mixture.
As bondo cures through a heat-generating chemical reaction, higher ambient temperatures will accelerate the process. That means the hotter it is, the faster your bondo repair will dry.
Heat can have a major impact on dry times, which means you may need to adjust your batch size when working in hot temperatures. A repair that would take 10 to 15 minutes to dry in moderate temperatures can dry in just 5 minutes in blazing summer weather.
The opposite goes for using bondo in cold weather. Cold temperatures will slow down the chemical reaction, increasing the cure time for your bondo application. If you’re having problems with your bondo curing too slowly, then a heat lamp or heat gun can accelerate the process.
In practical terms, this means you may need to modify the size of the batch you mix up to account for the temperature. In the midday summer heat, mixing up smaller batches makes sense, as this will allow you enough time to apply and shape the putty before it cures.
Ratio of Hardener to Putty
The other major factor that influences how long it takes bondo to dry is the amount of hardener added to the mixture. The more hardener you add to the mixture, the faster the putty will cure.
The general rule of thumb when using bondo is one inch of hardener per golfball-sized piece of putty. This ratio will allow the putty to remain workable for about 10 minutes before it becomes too gummy to shape effectively.
Adding extra hardener to the mixture will speed up the chemical reaction, causing the putty to dry faster than usual. On the other hand, using less hardener will slow down the chemical reaction, giving you additional time to manipulate the putty.
You might be tempted to decrease the amount of hardener in your mix to keep the bondo pliable for longer, but this can lead to the bondo never curing properly. It’s best to follow the manufacturer’s recommendation and stick to the normal ratio.
In terms of real-life usage, bondo dries pretty rapidly.
Let’s say you’re repairing rotted wood around a window and need to mix up batches of bondo. It’s a good idea to mix up smaller batches, as this will ensure you can use all of the putty before it becomes too gummy to apply.
This is especially true if you need to apply multiple coats of bondo. You might be tempted to mix up one big batch, but you’ll find that your batch will dry before you have enough time to apply a second coat.
For repairs deeper than ½ an inch, using multiple coats is generally recommended.
How Long Does Bondo Take to Dry Without Hardener?
As mentioned previously, the amount of hardener you add to the bondo mix will impact how fast it dries. But what happens if you don’t add any hardener at all? Will the bondo still dry?
Bondo is a two-part composite, which means neither part is any good without the other. Without the hardener, the resin may dry somewhat, but it won’t achieve that rock-solid cure that occurs when you use the correct ratio of hardener to resin.