After grinding the entire cabin top of our 36’ Union Polaris, we were happy to move on to smoothing out the holes and divots unveiled that we couldn’t grind flat. To do this, we chose to use 3M Marine Premium Filler. Along the way, we used a few project hacks that are worth passing on to help get this project completed rather quickly and keep it from getting too messy.

Before we get onto the hacks, a quick overview. Marine filler works in many ways like epoxy or cement. It requires the mixing of two parts – filler + hardener – and the moment they make contact, the chemical hardening process begins. This leaves you a short window of time of approximately 10 minutes to work in to ensure that you are able to apply the product to the areas you intend. For this reason, it’s best to mix it in batches and make only what you’ll be able to use within that time frame. Once hardened, it’s a force to be reckoned with on any flat surface but can be sometimes be cracked off if there are any thin or dangling pieces. When it cracks, it makes a clean break though you are not always able to predict where exactly that break will be. Nonetheless, if you drop a bit of pre-hardened filler, don’t assume you’ll be able to remove it from wherever it lands if you let it harden there. Keep this in mind as you read through the tricks below.

Have all the materials you need nearby

This is helpful for just about every project but can really save you time and even money when you’re completing a project using marine filler. Each time you have to go get something you’re missing, you end up letting excess filler harden on your mixing slab – and that means you’re losing money (this stuff isn’t cheap)! Below are all the items we gathered before beginning our project to make it a success:

  1. 3M Marine Premium Filler: comes in three sizes, a pint, quart, or gallon and has hardener packaged with it. We purchased a gallon knowing we would use quite a bit and likely end up needing filler for future projects as well.
  2. Cardboard cut into approximately 8-inch squares
  3. 12-inch mixing sticks
  4. Denatured alcohol
  5. Shop towels
  6. Box of nitrile gloves
  7. Thick paint scraper cut into 2-inch pieces
  8. Flathead screwdriver to open can of filler

A simple way to measure the filler + hardener

One thing that is sure to slow down your project is to meticulously measure the filler to hardener ratio each and every time you mix it using the exact ounce ratios on the can. To shortcut this process, we used a simple method:

    1. Put filler onto your cardboard. Using a wooden mixing stick, scoop the amount of filler you’d like to mix onto a piece of cardboard. Put it in one giant glob right in the center.
    1. Smooth the filler out into a circle. Begin to smooth the glob out into a circle with your mixing stick until it is about the thickness of a pancake.
    1. Draw a radius on your circular glob with the hardener. Squeeze the hardener beginning at the center of the glob and draw a thin line outward. The line should be about the same width as the hole the hardener is squeezed out of.
    1. Start mixing. As you mix the filler with the hardener, you’ll notice the filler begins to turn a light green. Once your entire glob of filler is a homogenous color, you’ll know you’ve mixed it completely.
  1. Begin applying filler to your desired location. At this point, the clock has begun ticking between the time the filler is malleable and when it will become hard like cement.

Use recyclable cardboard and wooden mixing sticks to combine the marine filler and hardener

We cut up recycled cardboard boxes into approximately 8 x 8-inch squares prior to starting the project. We used a fresh piece each time a new batch of filler was mixed. This made it so that when we mixed new filler, we didn’t have to worry about using marine filler that had chunks of already hardened filler in the new batch. Having a batch of filler that is hardening at the same rate makes it easier to apply and prevents any inconsistencies when you go to apply it.

Using cardboard is also quite cost efficient and quick to clean up since each piece can just be thrown away.  For similar reasons we used wooden mixing sticks to combine the marine filler with the hardener. We were able to break each paint stick in half, or sometimes even into thirds, so we didn’t have to use as many but still had a fresh one each time we mixed a new batch of filler.

Plastic paint scrapers make it easy to apply the filler where you want it

In contrast to their metal counterparts, plastic scrapers are far more flexible and when it comes to applying marine filler, we found this to be an advantage. We were also working with edges and inverted corners, so being able to bend the plastic scraper to press filler into the edge made it simple to get it all filled in. In addition, once the filler hardened on the plastic scraper, it was smooth and flexible enough to be able to break the hardened filler away from the scraper, making it like new again. A word of caution: we did try using a metal scraper and not only was it ineffective, we also ruined our scraper in the process because once it hardened the filler was incredibly difficult to remove.

Keep denatured alcohol and a shop rag nearby

Inevitably you will drop some filler that has already been mixed but has not yet hardened somewhere it isn’t supposed to be. When that happens, don’t panic. Using a shop rag with a little bit of denatured alcohol on it will help you wipe up any unwanted drops quickly and efficiently. If it’s not too much that fell, you will likely be able to use your shop rag again by simply folding it over and exposing a clean surface of the rag. Just remember it’s incredibly difficult to get hardened filler off of most surfaces without damaging them, so it’s important to attend to any drops before the filler has a chance to harden.

Regardless how long your list of boat projects is, we hope that using some of these tricks for using marine filler will make this particular project for you a little less painless. If you use any of our suggestions or have any tips to contribute, leave a comment below!

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