Epoxy Resin, Why did this happen?

[Cgarrett]

I am helping a friend replace a wood transom in a 14 foot aluminum boat.

Bought Marine grade plywood and used West Systems Epoxy to join the two pieces together to get the thickness of the previous one.

I brushed on the Epoxy to the back side that will be up against the aluminum transom part of the boat. When I brushed it on everything was fine and it looked smooth.

After several hours I looked at it and it dried with what I would describe as little bumps or pimples in some areas and is rough to the touch when you run your hand across it.
The coverage looks good still and there is no holes but still wonder why it is not smooth.

I have yet to do the side that will be visible and am looking for suggestions or a reason as to why this happened and what I can do to improve the look.

I used West Systems 105 Epoxy Resin and 205 Hardner.
5oz resin 1oz hardner and mixed well for around a minute.
Temp was indoor around 68

 

[jspano]

Re: Epoxy Resin, Why did this happen?

sounds like it set up too fast. i played around alittle bit before working on the finished transom. i know i used less harder than the package recommended.

joe

 

[PaulyV]

Re: Epoxy Resin, Why did this happen?

was that plywood Dry?? did it sit for 3 - 4 weeks before application? It may have been to wet

 

[piper_chuck]

Re: Epoxy Resin, Why did this happen?

Did you brush it on really thin? If so, a possible cause is that some of the epoxy soaked into the wood and some of the wood grain was raised. It's also possible some dust settled on the epoxy while it was curing. Whatever the cause, the cure is quite simple, rough sand it and apply another coat. I do this for most finishing where I want a smooth and thick enough finish.

 

[bananaboater]

Re: Epoxy Resin, Why did this happen?

A minute sounds like too short to get a really good mix. And I agree that the rough surface could be air bubbling to the sealed surface from the wood.

 

[Bondo]

Re: Epoxy Resin, Why did this happen?

After several hours I looked at it and it dried with what I would describe as little bumps or pimples in some areas and is rough to the touch when you run your hand across it.

Ayuh,... When you brush a layer of resin on virgin wood, it'll have air bubbles escaping from the wood during the entire hardening process...
Those little Bumps just need to be sanded smooth, before the next step...

 

[Cgarrett]

Re: Epoxy Resin, Why did this happen?

Thanks for the replys. You are dealing with a first timer on something like this.

We special ordered the Marine Grade Plywood from inside the Home Depot and when we picked it up it had cardboard on each side and we had it 2 weeks stored inside before we used it.

The surface was clean and dust free and I coated it in my basement with a constant temperature of around 68.

The combined amount of resin and harner was 6 ounces which to me was a small workable amount just right for the size of one side of the project. The length of the wood is around 4 feet and the height on each side is a foot and a half and dips down in the middle where the outboard mounts.

At the advise of the West Marine worker is where I got the mix for a minute for the small amount I was mixing. I felt the heat through the plastic mixing container as I was stirring.
The pot time on the container said 9-12 minutes so I wanted to make sure I had enough time to get the resin to the wood before it became un spreadable.

I was not stingy when I was spreading it on and got good coverage but did not want it dripping over the sides. I should add that I coated the sides but did not want drips all around the edges.

Now I will coat the side that will be visible from inside the boat.

If the same thing happens and we decide to sand it and apply another coat what sandpaper should I use?

All I can say is thanks again and it is great that we have these resources of information.

 

[Bondo]

Re: Epoxy Resin, Why did this happen?

If the same thing happens and we decide to sand it and apply another coat what sandpaper should I use?

Ayuh,... 80grit will knock the bumps down,+ scuff the gloss off in a hurry,...
180grit or 240 will also work, ya just gotta sand alittle harder, Longer...

 

[ondarvr]

Re: Epoxy Resin, Why did this happen?

Not only will there be bubbles, but there will be areas that soak up much more epoxy than others and its normal to apply more than one coat of epoxy to ensure the wood is sealed.

 

[thrillhouse700]

Re: Epoxy Resin, Why did this happen?

marine grade ply should be kiln dried, unless you bought it and strapped it to the roof of your honda civic hatchback and drove home in the rain it shouldn't need 2-3 weeks of drying.

 

[PaulyV]

Re: Epoxy Resin, Why did this happen?

interesting...always learning

 

[wizbang 13]

Re: Epoxy Resin, Why did this happen?

It's going to go against aluminum ?This is a pre coat ,so it dosen't matter anyway? I wouldn't jiggle the ratio as jspaono advised. Its tricky to get a smooth job with 'poxy.The 207 hardener does a bit better job ( different ratio).If it felt hot in the container, that may be too hot.

 

[erikgreen]

Re: Epoxy Resin, Why did this happen?

As has been mentioned, the bubbles were probably from outgassing. The best way to avoid that is to make sure your wood is dry, moisture content should be minimal before epoxy goes on.

What most people do is apply a layer of epoxy, then either apply a layer of thin glass cloth or mat, or sand and apply more epoxy or filler.

It's possible to get a nice smooth shiny finish with just one epoxy coat, but it's not guaranteed. If you're trying to waterproof the wood, use a thin layer of glass too, it'll last longer.

NEVER alter the mix ratio of epoxy to hardener. Epoxy is chemically different from poly resin. With poly you can use less "hardener" which is usually MEKP catalyst, to get a slower reaction.

Epoxy on the other hand does not use a catalyst... the hardener is the other half of the resin. If you alter the ratio of A-B, you're going to either have too much hardener or too little, so you end up with either extra part A or part B in the mix, which weakens the resin by changing its properties. At most you can be off by 10% and still get usable resin (this lets people have mixing errors) but you're not changing the kick or hardening time. That's mostly temperature dependent.

If you want a slower kick on epoxy (or faster) you have to use a different hardener. Usually the different hardener speeds also change the amount of hardener, but rest assured that's not what changes the speed. It's different chemicals.

Erik

 

[Cgarrett]

Re: Epoxy Resin, Why did this happen?

I want to thank you all for your knowledge and quick response.

It sounds like what happened is pretty common on a first coat so we will just litely sand down the visible side and give it another coat.