Learning Fin Construction Part IV: Sand, Sandin’ Away

In case you missed the last bit, it’s here: Part III: Hey! Cut it Out!
So we’re back at finishing up our first fin. At this point, we had added a bit of fiberglass to build the base up some to make sure we would have it a little thicker than the fin box so it could be “whittled” down to spec.
We hand sanded the surface of the fin somewhat, but my husband wanted to put a little vertical angle in the base part that goes into the fin box so the fit would mimic the dimensions we had taken of the professionally made fin. For this, we used the wet tile saw again.
I cut across the bottom horizontally to remove excess fiberglass overhang so there would be a level bottom for the vertical pass.
For the record…I thought the angle was so minimal I wanted to hand sand it down, but my precise better half wanted to try this…..
The jig my husband created was from factory cut wood and a couple of wall brackets. The angle was created by inserting two washers under each screw to “kick out” the wood surface the fin would lay upon.

Let’s give it a go!

Ok, so it worked alright, but there was some bounce in the fin off the wood even with clamping it down, so the diamond blade chopped a little into the base, but nothing that was a big deal. It was worth it in the name of science!
The front and back fin box holes to hold in the fin had been drilled, and the pin and bolt and screw fit these drill holes successfully.
After that, we were back to sanding again. From before, we had some low spots and slight bubbling in the epoxy that I wanted to sand out. I used sandpaper from 50-120 grit to fine out the surface imperfections as much as possible, plus fine tune the fin base to make sure it would fit correctly into the box. I did do some of the sanding on the base with a power sander with an 80 grit band just to make that go a bit faster.

Imperfections needing sanding out

More sanding! Not done yet!

After much hand sanding and eyeballing (I’m not as precise) I was happy with the fit, so we got ready to throw another layer of epoxy on.
This time, I was going to paint another layer on, but I did switch resin and hardener brands. I used an “extra slow” hardener that claimed a working time of 3-4 hours, but we still wanted to resolve the issue of bubbles in the epoxy. To remedy this, we dediced to create a vacuum chamber to put the epoxy mix into to draw out bubbles.
This chamber was made from PVC plumbing fittings (with a clean out top as a lid, and a toilet flange as a base!) and two ports were drilled into the PVC for the vacuum pump, and one for a relief valve so the mix could be removed quickly without waiting on the vacuum to decay.

PVC Chamber
Chamber hooked up under vacuum
Inside the chamber (small mix cup shown inside for reference)

This time, we mixed the extra slow cure and immediately placed it into the vaccum chamber under vacuum for 3 minutes at 25 mmHg. I removed the cup to check it after this time and it was still very cloudy with numerous very microscopic bubbles, but still not warm, so it was returned to the chamber for 5 minutes and 30 seconds. It was then removed, and here’s what it looked like:

Side view- slight layer of foam
View from top- bubbles are only on surface

At this point, the cup felt a little warm, so I left it out sitting for 1 more minute. I then proceeded to paint the fin with a coat. The mix went on without bubbling evident, so I was happy. we also fixed a little notch that was over cut from some some time ago with some fiberglass chop (cut up bits of fiberglass mixed into the remainder of the epoxy). The fin was then hung up to dry on a line from the ceiling. The full cure time reported is 24 hours. Here is a picture from about 16 hours into the dry time- the black tape is electrical tape to prevent the epoxy from dripping onto the base part. Other than the evident line that was left from the original attempt at infusion bagging with epoxy that kicked too fast, I’m happy with the result:

Yay!

Now, after it dries, we will throw on a gloss coat and hopefully take it for a ride (well, the husband will have to- I’m still on the injury list). Too bad the good waves already happened this weekend, but we’re not into zoos anyway!
So for the next infusion bagging attempt, we’re going to use the vacuum chamber to eliminate some of the bubbling in the epoxy. More to come, I’m sure…..

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