How To Thin Bondo? (Explained)

There are two main reasons why you may be interested in thinning Bondo. First, it could be because of improper storage. If you never stir your Bondo can thoroughly before and after use, it may harden after a while.

The other reason why people may want to thin Bondo is to get a thinner finish when using it as a filler. Both these reasons are valid. However, Bondo wasn’t meant to be thinned. So, is it okay to do it?

Before explaining how to thin Bondo, let’s answer the above question. Bondo isn’t designed to be thinned. It should be used as it is. The only thing you may have to add to Bondo is a hardener and nothing more.

Therefore, if you are contemplating to thin Bondo, chances are you did something wrong. Anyway, just because these two ingredients weren’t meant to be used together, that doesn’t mean you can’t.

A lot of people thin Bondo all the time. Sometimes it backfires, and in other situations, it works flawlessly.

How Do I Thin Bondo?

The best product you should use to thin Bondo is polyester resin. It makes Bondo softer and ensures an easier spread.

Polyester resin is the best choice out of all the ingredients DIY detailers recommend. However, remember when we said that Bondo wasn’t initially meant to be mixed with other products?

Thinning Bondo has its drawbacks. First, it increases the amount of shrinkage once it dries up. Shrinking is a menace when it comes to body filler.

If Bondo shrinks, it can fall off or even let moisture and air in, accelerating corrosion. This is a risk that you have to be aware of.

Another drawback of thinning Bondo using polyester resin is that it makes the surface harder to sand. If you have used body filler in the past, you understand the importance of sanding.

By making the surface difficult to sand, thinned Bondo can bring about many challenges in a painting project.

Thinning Bondo is a fairly easy process. As long as you have polyester or fiberglass resin, pour some, and you will notice that the Bondo will become softer. However, is it worth all the risks?

What about Using Less Hardener?

Not all painting projects are the same. There are times when you may need to thin Bondo to get a longer working time.

Hardener helps Bondo dry into a hard substance once applied to the car surface. You could be wondering, what if I use low amounts of hardener? Can that make it thinner?

Unfortunately, the answer is no. The body filler manufacturers have a valid reason why they often give you a Bondo to hardener ratio. If you use less hardener, the Bondo won’t be thinner.

But you are assured of an incomplete cure. Yes, it may not cure as fast as it would have when you use the correct amount of hardener. But at the end of the day, you will be left with a sticky mess.

Instead of resulting in strange tactics such as using too little hardener so that Bondo can be thinner. You are better off adding polyester resin to Bondo. It’s important to remember that this isn’t risk-free as well.

The Importance of Batch Tests

Undoubtedly, many things can go wrong when you thin Bondo. To avoid running into a weird situation, you should perform batch tests first.

This being a trial and error kind of situation, you don’t want to go all-in and regret it later. So as an intelligent DIY enthusiast, perform a batch test.

Before thinning the Bondo and using it in your painting project, do a quick test run on an inconspicuous part of the car.

Alternatively, find an old car and try to see the results you will get. It’s not always when things will go wrong. It’s therefore worth trying.

What about Plastik Honey?

If you have done some research on thinning Bondo, then you must have come across the product known as Plastik Honey. This is a product made by Evercoat, and its role is to thin Bondo.

It is a much more effective alternative to ingredients such as acetone or alcohol. Plastik Honey, similar to polyester resin, will thin Bondo.

But, you can expect some shrinkage issues later on. This is unavoidable no matter what ingredient you use to thin the Bondo.

Can Thinner Prevent Bondo from Drying Up So Fast?

If you are thinning Bondo to prevent it from drying up so fast, you may have to take a step back and re-evaluate your strategy. Thinning Bondo has no relation to how long Bondo takes to cure.

Hardener is the ingredient that determines how long this process takes. Too much hardener will cause Bondo to dry up fast and vice versa.

There are other reasons why Bondo could be drying up so fast. One of the most common ones is the temperature. If it’s too hot outside, Bondo will dry up pretty much quicker than if it was cooler.

If you don’t want Bondo to dry up fast, you should use less hardener and work in a cool, dry area.

Under a tree, you can get enough shade that will keep the scorching temperatures away. You should also know that different fillers have varying curing times.

How Do I Prevent Bondo in a Can from Drying?

Stored Bondo that has been on the shelf for a while can start to dry. The drying often occurs at the bottom of the can. This is very common because Bondo is a two-part putty.

The plastic component often sinks to the bottom of the can, and if you don’t usually mix it in the correct proportions, the likelihood of this happening is very high.

If you want to prevent Bondo in a can from drying, you can always add polyester resin. This will keep it in a gooey state for the longest time. The other tip is to practice proper storage techniques. This involves mixing it after every use.

Does Thinning Bondo Make It Harder to Sand?

Using too much thinner can make it hard to sand over the filler. Thinned Bondo is often harder than regular Bondo; therefore, you may have to use a lot of elbow grease to get a smoother finish.

To avoid running into Bondo issues, you should avoid adding any foreign elements to it. Bondo works perfectly as it is. The only ingredient you are allowed to add is a hardener.

What Other Ingredients Can I Use to Thin Bondo?

If you don’t have polyester resin or Plastik Honey, you can always use acetone. The problem with acetone is that it can damage Bondo when used in large amounts.

This is why we don’t recommend its use. However, considering that acetone is the most popular ingredient amongst the ones listed above, you can give it a try.

Don’t forget the importance of trying it out on a small amount of Bondo first. This will allow you to experiment and determine whether it’s safe for use.

Does Thinning Bondo Make It Easier to Apply?

When you thin Bondo, it becomes less gooey. As a result, you can spread it much more easily on surfaces. Bondo, when it’s in its original form, maybe a bit thick. But it’s still easily applicable. You may be interested in a thinner finish.

But, you can still achieve the same with ordinary Bondo. We have already seen the side effects of thinning it. Yes, it can make applications easy. But what will you do when it starts to shrink?

It’s better to struggle during the application process and have the Bondo stay put for years than undergo an easy application process only you to regret it a few months later.

Does Heating Bondo Make It Thinner?

You will be surprised at how creative DIYourselfers can be. We came across a post on a particular forum that claims heat will thin Bondo. As odd as this may sound, heating Bondo can help thin it.

However, you need to be cautious because it will harden completely when it cools. So, ensure you use it as soon as it starts to thin. This way, when it dries, you will be done with the project.

Is It Worth Thinning Bondo?

Body filler is quite affordable. Therefore, there is no reason you may want to thin Bondo that has dried in a can unless you are miles away from stores.

What makes the most sense is if you want to use it when it’s still soft, or if you prefer working with a thinner substance.

When you thin Bondo by adding chemicals, you change its physical composition. This prevents it from functioning as it usually would.

It may shrink sooner than it does. Or you may run into a lot of difficulties when sanding. Bondo works exceptionally well without any additives except for hardener.

If you are experiencing any challenges, always confirm the required mixing and application methods of Bondo.

At times, hiring a detailer to work on your car can save you the hassle of thinning Bondo and dealing with the adverse side effects.