Sunday, May 5, 2013

Fiberglassing Sonotube

I finished applying resin to the outside of the tube today:

Here are the details of how I supported it between two chairs, with pretty massive amounts of weight on each chair.  (If it looks precarious -- it is.  At one point when rolling the resin, it started rocking enough to fly off.)

Here you can see the color difference between initially wet and dry:

As the reaction takes place (a peroxide hardening agent) it turns the resin brown.  Fortunately, after sanding, I will paint it.

While I decided that it did not truly need to have fiberglass cloth embedded in it, I thought that for the end of the tube where the mirror cell will be mounted (and thus where the load will be most severe), putting in one layer of cloth wasn't a bad idea.  This both increases stiffness, and provides an additional layer through which bolts will go.  When I drill the holes for the various parts, I will soak the exposed Sonotube material with resin to reinforce support.

It looks awful, but once sanded, it will be an even edge.  Unlike some of my previous fiberglass experience, I managed to get all the air bubbles out of it.  This time, instead of using a brush, I used a roller.  In addition, the last time I made a fiberglass tube, I used too much resin.  I applied a thick coat of resin, then the resin-soaked cloth, and not surprisingly, the cloth kept slipping.  This time, I waited until the first layer of resin had become sticky, lightly soaked the cloth in the resin in the paint tray, then applied it to the sticky layer.  Then I applied a bit more resin to the top of the cloth, squeezing out air bubbles without causing the cloth to slip.  I think I am getting the hang of this.

It has been sitting for about six hours now, and the exterior is still tacky in places, but there is absolutely no question that the tube is much stiffer than before.  Sonotube by itself would deform slightly if I pressed down on with my hand, enough that I could see it deform if I looked closely.  It appears that even though it is not completely cured, there is no obvious deformation.

How much weight have I added?  A quart can of this resin weighs a bit less than 2.5 pounds, and I have used the first can.  My guess is that the hardening reaction and evaporation causes loss of volatiles; in addition, not all the resin actually ended up on the tube.  Some hardened in the reaction vessel; some in the paint tray; some on the brush.  (It is much easier to remove the brush from the roller while it is still wet.)  I would be surprised if there is actually two pounds added to the tube.  For this stiffness, a 28 pound tube is just fine.

I have class tomorrow night, so I won't have a chance to return to this project until Tuesday night, by which point I expect it to be rock solid and dry.  At that point, I will drill all the required holes, put some resin on the inside of the holes, and give it another night to harden.

I still need to revise the mounting scheme for the mirror cell.  When I designed this mirror cell, I expected to put it in a lower cage, so the bolts go into the tube and you tighten the nuts from the front of the tube.  That won't work on 74" long solid tube (unless I can find someone with six feet arms or who stands about 18" tall).  Instead, I will remove the clips that point forward and reattach them pointing backward.  This will let me access the fasteners from the rear.

I still have not abandoned the idea of fiberglassing the lower cage that was bending under load, but partly because I am curious, and partly because it will make it easier to sell the rest of the truss tube assembly if I have something that is stiff enough to not bend.  Unfortunately, fiberglassing the lower cage will make even more of a weight advantage for the solid tube than it had before, and portability just isn't that critical.

I should explain that part of what motivated the concern about portability is that Big Bertha in the form that I first acquired it was too large to move in my wife's Equinox.  I actually had to borrow a friend's Ford Expedition to get it to the new house.  A solid tube version is still something that can fit into the TrailBlazer that we have now.

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