What Body Filler to Use on Fiberglass (14 Options and Guide)

Getting the trailer fixed. Many RV and trailer owners become DIY experts. Instead of paying the professional, they do the work themselves. This includes working with fiberglass. It is not a difficult project to undertake and all you need is the right auto body filler to make the work look good.

What body filler to use on fiberglass? Fiberglass and resin are great for small holes but when you have a large area to cover, it is best to use some fiberglass and resin, then smooth out the surface with a good Bondo. Both are made from similar ingredients so they work well together.

To learn more about what body filler to use when working with fiberglass, just keep reading our article. It has the information you need to do a good job on any repair work you are doing on your all fiberglass trailers or RVs.

What Body Filler To Use On Fiberglass


There are a lot of products you can buy to help you smooth out any ripples in your fiberglass. Some are more expensive than others and while the cheaper ones are not bad, the more expensive ones are better in this case.

The more expensive products are less prone to the tiny air holes you find in some filler products when you go to sanding the stuff smoothly. Some of these products may come in different colors and one comes in white, gray, and black.

The names of these high-end fillers are RageGold, RageUltra, Z-Grip or Evercoat Lite Weight, Kitty Hair, and Tamco. You have a lot of choice and trial and error is going to be your best teacher. Every person has had different results due to their handyman skill levels.

We have said constantly on different projects to be patient and take your time. This advice applies to working with fiberglass and body filler. Also, make sure to read the instructions carefully as you will want to wait the full curing time before doing the next step in completing the project.

Application is key to the whole process so make sure you understand what you need to do before you do it. It will be difficult to fix your repair if a mistake is made.

Filler For Fiberglass Repair Options

One thing you should know before we get started, polyester resin does not adhere well to cured polyester. That is why some people use the first suggestion more often than not. A good epoxy is a great filler when working with fiberglass.

The biggest difference in using epoxy or polyester resin is that the latter product can have its cure time adjusted, epoxy cannot. Also, epoxy comes with a wax-like ingredient that seals the epoxy and lets it cure. This wax coating has to be removed before you can paint.

Polyester does not adhere to epoxy so once you use the latter filler, you have to continue using epoxy or your project may fail. Another product you can use would be a high-build primer.

This product comes in a spray can and all you have to do is follow the instructions on the can. That should make things easier for you. This spray filler gives you a thin coat that sands easily and also provides you with a great finish for you to paint over.

Body filler is more for appearance than anything else. it does not add strength to the materials. All it does is smooth out the surface so you do not have waves, ripples, or scratches on your walls.

When you are working in areas that won’t be seen, you can skip this step and save yourself time, money, and energy. Unless you are a perfectionist and need to have everything inch looking perfect.

Can I Use Car Body Filler On Fiberglass?


Yes, this is possible and some of the brands we mentioned earlier are car body fillers. The general term for all auto body fillers is bondo and that is how most body men refer to the products they use. They don't use specific brand names for the most part.

The fiberglass type that you shouldn’t use auto body filler is the SMC panels. Whether those panels are in your trailer or RV is another matter. You should identify the type of fiberglass you have before you go out and buy the bondo.

On top of that, one expert says that for top fiberglass repairs, you should only use what is called vinylester. This is a high-strength filler and unfortunately, at the time of this writing, there is only one brand that has this product on the market.

The product and brand are called 3M High Strength Repair Filler. If there are others by now, we have not come across their names during our research. If you don't need high-strength filler, and most won’t as the fiberglass provides the strength you need, then regular bondo will work.

Regular bondo cures quickly, can be shaped in minutes, and is designed to not shrink while permanently bonding to the surface you apply it to.

Can You Use Spot Putty On Fiberglass?

Yes, this is a possibility but spot putty is not made for large holes or waves, etc. it is best used on small holes, and can work well on those pinholes you get when sanding larger holes filled with bondo or other body filler.

We do not know all the brands that make this spot putty but one brand Bondo® Glazing & Spot Putty and another is 3M™ Body Repair System. When using spot putty, the application process is quite simple.

All you need to do is prepare the surface. This includes making dents smaller, removing old paint or any rust in the area if there is any. Then the second step would be to fill those small holes.

The filling includes smoothing and shaping before you do the third step which is painting. You should use a good smoothing primer before you paint. Then when that is cured, paint away.

The final step will be to finish the area with a sealer or polishing clear coats. Make sure you give each stage the time it needs to dry completely before moving on to the next stage.

There may be other brands out there that are just as good as these two. Pick the one you are most comfortable with using. The use of spot putty is not that complicated and each brand should have similar simple stages to follow.

Can You Put Bondo On Fiberglass?


Yes, this is possible. there may be specific fiberglass panels that you cannot use bondo on but those situations are rare. Regular bondo is safe to use and is recommended for many fiberglass applications.

You can also put fiberglass over bondo if the situation calls for it. You would need to rough up the bondo a bit using an 80 grit piece of sandpaper but it will adhere to the bondo. There is a reason why these two products are so compatible.

Both fiberglass and bondo use styrene resin as a base. In other words, the two products are compatible because they use similar ingredients. Not all of them are the same as fiberglass has glass fibers as a filler and bondo uses talcum powder.

There is a hardener that is needed to help the styrene resin cure but you have to be careful when adding the hardener. Too much and the bondo hardens before you get it out of the can.

There is no restriction on how little you need to use. The less hardener only means the longer the bondo will cure. it will set up eventually and you may have to wait 24 hours before it does.

Remember body filler is only for texture, it is not for adding any strength to the repair. That duty falls on the shoulders of the fiberglass.

Bondo Or Fiberglass Filler?

The product you use will depend on if you need strength or looks. The former will help with the looks but not the strength. The latter will provide you with the strength but it is short on creating good looks to your end repair.

Keep in mind that that the term bondo is a generalized term. All fillers are listed as bondo but there is only one Bondo brand. It is like all tissues are called Kleenex but not all tissues are made by or are Kleenex. There are different levels of quality of tissues and the same applies to bondo.

However, you may not need to answer that question because we are seeing many products where the bondo has fiberglass strands added to it. 3M makes Bondo glass-reinforced filler as well as other similar products.

These products are supposed to remove the need to add fiberglass to your repair. The Bondo brand also makes this type of product called Bondo-Glass. Its ingredients include fiberglass strands and remove the need to add for backing strips, fiberglass resin, or cloth.

These products and similar ones made by other brands are worth considering when doing small repair jobs. The second one listed here is supposed to be ready to sand in 20 minutes, includes the hardener, and is waterproof.

Using Bondo Over Fiberglass Resin


For small repair jobs, this may be your best option. Not only do you save time and energy, but you can save money as well. But using bondo in large holes is not a good idea. Bondo is used to help the repair look good.

This product is not used when you need the hole to have a lot of strength. It is best to use fiberglass resin in the large repair areas so that your trailer or RV wall, etc., has a lot of strength to it. Then use the bondo to make the repair look like the area has never been repaired.

Also, if the repair is in an out-of-the-way spot that will never be seen, you can use fiberglass instead of the bondo and leave it looking rough. How you use these two products will depend on their location and your preference.

Or you can use the products we mentioned in the previous section. Those are a combination of the two repair materials and may save you a step or two in the repair process. It is worth considering when you only have small repair projects to take care of.

If you are not sure, ask an expert what they think is the best way to go when making fiberglass repairs.

How To Apply Bondo To Fiberglass

Application is not hard to do. There will be some sanding to do so that the fiberglass will be roughed up enough for the bondo to adhere to it correctly. Next, you should tape off the area not being repaired as you do not want any of the dust particles from the sanding to reach those areas not being repaired.

Then after the surface is ready, you mix the bondo with the hardener. Make sure to follow instructions carefully. You can use too little hardener but you cannot use too much. Once the hardener is mixed in you should use a plastic spreader.

That tool may not last as long or give a great edge to the bondo when you are done but it also won’t scratch any surface. Then let the bondo dry before sanding that surface down.

This repair work is not hard to do it just needs a careful touch. The amount of hardener you use will either speed up the curing time or slow it down. One thing you do not want is a very quick hardening time as then you won’t be able to apply the bondo.

When To Use Fiberglass Body Filler


Some of these fiberglass body fillers are not waterproof. That means you do not want to use them when you are doing exterior work. You will want waterproof materials when working on the outside of your trailer.

Some products will say water-resistant but that characteristic is not the same as being waterproof. So be careful in selecting the fiberglass body filler. Then most fiberglass body fillers are just bondo with fiberglass reinforcement.

Not all are and fiberglass body fillers are used when you need lots of strength in a given area. This strength is found in the long strand versions. However, the long strand is difficult to apply smoothly and is usually reserved for must-have strength repair situations.

You won’t need to use this type of body filler all the time. You will have to assess the area and determine if it needs a lot of strength or just good looks with a little less strength.

How To Use Body Filler On Fiberglass

One reason why many trailer and RV owners turn to fiberglass and bondo body fillers is that purchasing replacement fiberglass panels is very expensive. Those with all fiberglass trailers do not even have that option and replacing their trailer is even more costly. Here are some steps to using body filler on fiberglass:

1. Find the hardener tube then find a clean piece of glass or plastic to do the mixing of the bondo. You cannot use cardboard as it is absorbent and will disrupt the chemistry of the mix.

2. Mix the bondo so that the chemicals inside are evenly distributed.

3. Knead the tube of hardener so that it is even mixed as well. Do not mix the two products until they are mixed evenly.

4. Pour or you can scoop some of the bondo on the glass or plastic mixing palette. Next squeeze out a single line of the hardener across the top of the bondo. Here less is more works and you do not want to add too much hardener.

The more hardener you use the faster the bondo will harden.

5. Once that single line has been added, mix the two products together until they are the same color. The color should be uniform.

7. Using a plastic spreader, spread the mixture over the area. You should not go more than 1/4 inch thick. Make sure to only wipe the filler in one direction. using multiple directions creates small air bubbles

8. The drying time can range, depending on how much hardener you use. it can be as little as 5 minutes or as long as 20. Sometimes, it can take 8 hours.

9. When dry, you can sand and paint.

Some Final Words

Some good products on the market will work for your fiberglass repair project. Some will cost a lot and others will be more affordable. In this case, the more expensive is usually the better product to use but not always. When in doubt ask an expert.

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