If you need to fill or repair a variety of exterior and interior surfaces, then Bondo is always a good option to consider. That said, there are some things about this versatile filler and repair material that you need to know.
One of the most important aspects here is drying, specifically how long it takes this substance to dry, how long you have to wait to sand or paint over it, and what affects the drying time. So, how long does Bondo take to dry?
How Long Does Bondo Take to Dry?
One of the things that stands out about Bondo, when compared to other wood fillers and surface fillers, and why it makes for such a popular option, is due to its very fast drying time. If you use this product according to the official directions from the manufacturer, it should dry in about 10 to 15 minutes, at least enough to form a hard surface that can be sanded.
Now, what does need to be made clear is that while this filler does dry as moisture evaporates out of it, it does actually harden through curing, which is a chemical reaction that happens between the Bondo and the hardener being used, not unlike two-part epoxies.
Okay, so technically speaking, this substance is actually a specific type of two-part epoxy, which consists of the resin and the hardener. So, in terms of curing, it will take Bondo about 45 minutes to one hour to cure, at least enough so that it can be painted, with a full cure taking several hours to achieve.
How Long Should You Wait for Bondo to Dry Before Sanding It?
If the Bondo-to-hardener ratio is correct, and the room temperature is moderate, the substance should be dry enough to sand down in about 10 to 15 minutes after application.
That said, if you want to be extra safe, so you don’t wreck your filler job, you could wait as long as 20 or 25 minutes before you attempt sanding it.
How Long Should You Wait for Bondo to Dry Before Painting Over It?
Keep in mind that not only should Bondo be dry to paint over it, but also mostly cured. In this case, you should allow about 45 minutes to one hour for the filler to cure and harden enough to paint over.
Once again, this is only if the Bondo-to-hardener ratio was proper and if the room temperature is moderate. If you are unsure, you may want to wait as long as 75 or 90 minutes before you try painting over this material.
Does Bondo Harden Without Using Hardener?
The simple answer here is that no, Bondo will not harden if you do not use the hardener. Remember, this product is a two-part epoxy, with both of those parts being equally important. One part is going to be the filler itself, which in this case is the resin that will then harden and form a solid material, a specific type of thermoplastic.
Then, the second part is the hardener, which acts as the catalyst that begins the chemical reaction of the curing process. The only way Bondo can harden is if it is mixed with the hardener that will begin the curing process.
If this substance hardened on its own, every time you open the container, you’d be faced with a rock-solid mass. So, yes, you do need to mix the hardener with the Bondo, using the correct ratio in order for it to cure and harden.
3 Things Affecting Bondo Drying Time
There are three main factors that will affect how long Bondo takes to dry, including the amount of hardener used, the ambient temperature, and how thick the layer is. Let’s take a look.
1. The Bondo-to-Hardener Ratio
The first factor that will determine how long Bondo takes to dry is how much hardener you add to the substance. If you add more hardener it will dry and cure faster, and it will dry and cure slower if you add less. The official instructions say that you should add roughly one inch of the hardener for every golf ball-sized piece of Bondo being used.
2. Ambient Temperature
The next factor that affects how long this filler takes to dry and cure is the ambient temperature. If we are talking about normal room temperature, so around 20 degrees Celsius, it will take about 10 to 15 minutes for it to dry, and about 45 minutes to one hour to cure. However, if we are talking about super-hot summer days, it may take only 5 minutes to dry, whereas on really cold days, it may take up to 30 or 45 minutes to dry, and up to several hours to cure.
3. Thickness of the Layer Being Applied
The other factor to consider here is simply how thick the layer of Bondo being applied is. The thicker the layer and the more you apply, the longer it will take to dry and cure.
How to Speed Up Bondo Drying Time
Let’s take a quick look at how you can make this substance dry faster. That said, speeding up drying time may affect the quality of the cure and the final product, so it’s not the first thing we would do.
After all, Bondo already dries and cures so fast that risking the integrity of the material by speeding up the drying time is not worth it.
1. Add More Hardener
The easiest way to speed up the drying time is to add a bit more hardener to the mix. Now, this is going to make working with it difficult, as it will dry exceedingly fast.
2. Heat Things Up
Using Bondo on a hot day will also help speed things up, or you may want to use a heat gun, heat lamp, or even a hair dryer to add some heat to the mix.
3. Keep Coats Thin
If you want the product to dry quickly, and you are doing a deep repair, sticking with thin coats that are no thicker than ½” is recommended.
Summary
There you have it, folks, you should now know everything there is to know about how long it takes Bondo to dry and what affects its drying time.
You might also want to learn how long wood filler takes to dry and how long wood putty takes to dry.