Learning Fin Construction Part II: Glassing Shouldn’t Suck

Of course, I’m going to do the Norm Abrams thing and remind everyone to follow all safety precautions before working with tools or materials that may be hazardous!

So we left off Part I with three pieces of balsa ready to glue together to make a simple core for the fin. Each side piece was cut just a 1/4″ smaller than the center, in order to give some dimension- hopefully.

Ready to glue one side to center using Carpenter’s glue
Lining up one side with the center piece to ensure the 1/4″ mark is aligned

After both sides had been glued onto the center, like a sandwich, I once again covered the core with cling wrap and weighted it with books to ensure that it would dry thoroughly. I let it dry for a day. The core was gently sanded again to blend the edges together.
Meanwhile, in the garage, my husband had been calculating using a dial caliper how much 7 oz. (approximately) fiberglass cloth (7 oz. cloth means the cloth weighs 7 ounces per square yard) would be needed, using the pro-made fin as a guide. I had drawn a template that was 1/4″ larger all around than the balsa core, and included the section that fit into the fin box, 7/8″ longer than the bottom of the balsa core, and made as a simple rectangle. Since this section that fit into the fin box was going to be made entirely of fiberglass, it was going to take A LOT of cloth to build it up.
First, he determined that he would need  a certain amount of full fin pieces on each side of the core to build up the fin sufficiently. He drew these out on the fiberglass cloth and cut them out with scissors, wearing gloves, of course, and using a shop vac to clean up any excess material.

Drawing out the templates on the fiberglass cloth and cutting out the shapes

Of course, more pieces were needed for the bottom rectangle section to get it built up to the desired thickness. Ultimately, there were 9 layers on each side of 7 oz. cloth cut and laid onto the core, and an additional 25 layers just for the the bottom area that fits into the box to bring it up to thickness.
Next, we were ready to set up the bag for the vacuum. The bagging material allows for the item inside to be held under vacuum so that the fluid epoxy may be drawn into the layers of fiberglass, bonding them together for strength and creating a hard laminated piece. This info sheet by West Systems has a really good walkthrough of Vacuum Bagging (Link).
The plastic bagging material can be cut to whatever size is needed, but will need to have an access port for the vacuum to suck the air out of the bag, as well as an access port to draw the epoxy mixture fluid into the bag while under vacuum. Also, we would need to ensure that the bag is sealed tight everywhere else so we can make sure we’ve got a good vacuum. I like to think of this process as using one of those “As Seen On TV” Space Saver bags, where someone puts their lifetime collection of bad Christmas sweaters into the bag and sucks the air out by a home vacuum cleaner to reduce the size for storage and/or hiding.
Before sealing up the bag, the fin core and layers were carefully laid in the bag, and a product called peel ply was used as well to insure the bag would not stick to the laminate. We also placed this batting material around the fin (looks like the stuff I use in blankets) to help wick the epoxy fluid into the fin area evenly.
To keep the bag sealed, we used this double sided foam black tape that came with the vacuum bagging setup. It works pretty well, and we taped up three sides (the fourth was a fold) this way. When it came to the vacuum port and epoxy port, though, the tape didn’t seem to wrap around the tubes very well, so some Sta-Put plumber’s gray putty was very useful in sealing leaks, and wouldn’t contaminate the bag’s contents. In fact, we drew a vacuum, and listened for any leaks while watching the gauge on the pump we were using to pull a vacuum while the epoxy tube was clamped off. We were able to achieve a clamp force of almost 25 inHg (inches of mercury). This means we were able to make the force that the bag exerts on the fin around 12 psi. A perfect vacuum in the bag would mean we would have to have 14.7 psi (regular atmospheric pressure, or 1 atmosphere) clamping force, or almost 30 inHg! Just not gonna happen in the garage lab.

Getting ready to attach the hose to the vacuum outlet

Next, it was time to mix up the epoxy. This is a great time to be donning the safety glasses, nitrile gloves, and mask. Also, make sure the room you’re working in is ventilated. Read the instructions on the mix!
We got resin and slow set hardener. The slow set we got is typically supposed to take around a half an hour to “kick”, or set. Or so we thought.
The mix was 2 parts resin to 1 part hardener. We used measuring cups from the hardware store for mixing, but pouring exact amounts from the jar is tricky. If you can get a hold of a pump dispenser for the resin and hardener, it will make your life easier. Otherwise, you can use these multi measure cups like this:

These cups are handy- when you get the code down. I used the 4:2:1 ratio section on the side for mixing. This means that I filled the resin (I needed 2 parts resin) up to the “4” in the FIRST column in black. I then took the hardener and filled the cup to the blue “4” in the SECOND column. This would give me the proper proportion for the mix.

The 4:2:1 section I used with the 3 columns shown

I used wood sticks to “honey-drip” to get right amounts, and was careful not to leave stuff on the sides of the cup to throw off my measurements. I mixed the parts together with a wood stick thoroughly, but quickly. We flipped on the vacuum pump, unclamped the inlet hose with the hose placed into the cup.

Epoxy drawing into the bag and fin under vacuum

The epoxy started to draw into the bag rather quickly, sucking up much more epoxy than anticipated. We had some more already mixed ready to go with the both of us working, so we switched out cups immediately. We noticed, though, that the epoxy seemed to stop drawing up about halfway up the fin and the cup became very hot- even melted the plastic cup. This all happened in less than ten minutes- the inlet tube became rock solid, the bottom manifold was hard, and we had already run out of time. What happened?
We were supposed to get 30 minutes to work with this mix, but it kicked WAY too early. We theorized that maybe:
a) The newer mixture may not be as slow set as the previous mix he used in the past,
b) We got the wrong mix,
c) We were working in conditions that were too hot and humid in the garage, causing it too kick too early.
We rearranged the setup and put the inlet for the epoxy at the top of the bag this time, so that we could try to get the other half of the fin wet. This worked, but of course, left some dry spots in the glass when we removed the bag and peel ply. On top of that, we had a big nasty hard rock of epoxy attached to the bottom of the fin.
We used a small plastic squeegee to spread some mix on the dry spots which kind of blemished the nice smooth surface we were hoping to achieve with the vacuum. The edges of the fin, of course, were a little rough, but that could be taken of with sanding, we hoped, as well as the surface imperfections.

Next phase…

Part III: Hey! Cut it Out!


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