Body Filler not Hardening. What Can I Do to Catalyze the Process?

The curing process of body filler is crucial to the overall strength and durability of your repair. If the filler does not cure properly, it will be significantly weaker than it needs to be and can lead to further problems down the road. In this article, we’ll discuss what you can do to catalyze the curing process of body filler so that your repairs are strong and long-lasting.

What is a Body Filler?

Body filler, also called bondo, is a type of plastic putty used to fill in dents and scratches in vehicles and other objects. This substance commonly finds use in industries such as automotive body repair and boatbuilding.

Bondo is composed of a resin and a hardener, usually in the form of a cream and powder. When these two substances are mixed, a chemical reaction takes place that causes the filler to harden.

Now, there are two main types of body filler commonly found in hardware stores and auto body shops: polyester filler and urethane filler. In the following, let’s take a closer look at each type and see how they differ and what are the main advantages and disadvantages of them both.

Polyester Filler

Polyester filler is the most commonly used type of body filler. It is made from a resin that is mixed with a hardener, usually in the form of a cream and powder. When these two substances are mixed, a chemical reaction takes place that causes the filler to harden. The main advantage of polyester filler is that it is very easy to use. All you need to do is mix the two substances and apply them to the surface you want to repair. Another advantage of polyester filler is that it is very strong and durable. Once it dries, it will be able to withstand a lot of wear and tear. The main disadvantage of polyester filler is that it takes a long time to dry. It can take up to 24 hours for the filler to fully harden. Another disadvantage of polyester filler is that it is not very flexible. This means that it can crack easily if it is not applied properly.

Urethane Filler

Urethane filler is another type of body filler that is made from a resin that is mixed with a hardener. However, unlike polyester filler, urethane filler is much more flexible. This flexibility makes urethane filler ideal for repairing surfaces that are subject to a lot of movements, such as car doors and hoods. The main advantage of urethane filler is that it dries very quickly. It can take as little as 30 minutes for the filler to harden. Another advantage of urethane filler is that it is very strong and durable. Once it dries, it will be able to withstand a lot of wear and tear. The main disadvantage of urethane filler is that it is more difficult to use than polyester filler. This is because urethane filler requires special equipment, such as a respirator, to apply properly.

What types of projects is body filler used for?

Body filler is a commonly used substance that can be used on a lot of materials such as wood, metal, plastic, and fiberglass.

Here are some examples of repair projects where applying body filler can be useful:

  • Filling in dents and scratches on vehicles
  • Filling in holes in walls
  • Filling in cracks in wood furniture
  • Filling in gaps in seams
  • Filling in gouges

How does body filler harden?

Body filler hardens when it is exposed to air. The exposure to air causes a chemical reaction that causes the filler to harden. Once the filler is hard, it can be sanded down and painted over. Now, what if your body filler isn’t hardening?

There could be a lot of reasons why your body filler isn’t hardening. In the following, we will look at some of the most commonly cited reasons and offer solutions on how to harden your body filler.

Common reasons for body filler not hardening

Body filler not properly hardening is quite a common problem. There are several reasons why this may occur.

Before we dive deep into some solutions, let’s first look at some of the most common causes of body filler not hardening:

  • The ambient temperature is too low

Body filler needs to be applied at a temperature of at least 70 degrees to cure properly. 70 degrees is a common room temperature, so if you’re working in a garage or outdoors, be sure to take this into account.

  • The humidity is too high

In most cases, you will not run into this issue. Still, it is worth pointing out that if you work in an area that is humid enough for the material to be damp to the touch, it will not cure as intended. For example, if you work outdoors in a tropical climate or a sauna, the humidity will be too high and the body filler will not cure.

  • The product was not mixed correctly

This is a common mistake that people make when using body filler for the first time. If you get the mixing ratios wrong, the body filler will not cure. Be sure to follow the exact instructions on the packaging to avoid this mistake.

  • You did not use a hardener

If you are using a two-part body filler, you must mix in the hardener that comes with it. Without the hardener, the body filler will not cure.

  • There was not enough heat

In some cases, simply applying more heat to the area where the body filler was applied can help to catalyze the curing process. This is because the hardener in two-part body fillers is activated by heat. So, if you find that your body filler is not hardening, try using a hair dryer or heat gun on the area for a few minutes.

  • Old or contaminated body filler

If you are using old body filler that has passed its expiry date, it may not harden. The same goes for using body filler that has been contaminated with something like water or oil. In both cases, it is best to start over with fresh material to avoid any issues.

These are just some of the reasons why body filler may not harden. If you find yourself in this situation, don’t panic. Just take a look at the possible causes and try to narrow down the problem.

Once you have figured out the issue, you will also obviously want a solution. In the following, we will look at what you can do to catalyze the curing process of body filler.

How to harden body filler?

If you find that your body filler is not hardening, there are a few things you can do to try and catalyze the process.

  • Heating up the area

The first thing you can try is to apply more heat to the area. As we mentioned before, the hardener in a two-part body filler is activated by heat. So, if you find that your body filler is not hardening, try using a heat gun or even a hair dryer on the area for a few minutes.

This may help to kickstart the curing process. Keep in mind that a hair dryer is not designed for such a process, which means that it may overheat quickly. So, be sure to keep moving it around and don’t let it stay in one spot for too long.

  • Pressure to the spot

Another thing you can try is to apply more pressure to the area. This will help to push the body filler into the cracks and crevices that you are trying to fill. By doing this, you will also be helping to spread out the heat more evenly, which may help with the curing process.

If all else fails, you can always try mixing in a little bit of hardener from a new tube. This should only be done as a last resort, as too much hardener can cause the body filler to become brittle.

  • Hardening accelerator

In some cases, using an accelerator may also help. An accelerator is a product that is designed to speed up the curing process of body filler. It is typically used in cases where the temperature is too low for the body filler to cure properly. However, it can also be used to catalyze the curing process in other situations.

When using an accelerator, it is important to follow the instructions on the packaging carefully. In most cases, you will simply need to add a few drops of the accelerator to the body filler before mixing it.

Once you have mixed it well, you can then apply it as usual. One thing to keep in mind with accelerators is that they can cause the body filler to become hard very quickly. So, if you are not careful, you may find that the body filler sets before you have had a chance to smooth it out.

  • Redo the job

If you accidentally used an expired product that does not act exactly as intended, your best bet is to buy some new body filler and start over. The same goes for if you accidentally contaminated the body filler with something like water or oil. In both cases, it is best to avoid any potential issues by starting from scratch with fresh material. To get rid of the residue of the contaminated body filler, just sand it off with some sandpaper before starting work with a new product.

Final words

As you can see, there are a few things you can do if you find that your body filler is not hardening. In most cases, the problem can be easily identifiable and solvable, so you should have no trouble getting your body filler to cure properly. We hope that this article was helpful to you and that all your projects go smoothly from here on out!

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