Wednesday, May 8, 2013

Fiberglassing Sonotube, Continued

I noticed that the resin was really not setting terribly quickly, even with heaters running under the tube.  I am beginning to think that the problem was that I didn't add enough hardener to the polyester resin. Also, this discussion of use of polyester resin for fiberglass makes the point that these resins often include some wax which rises to the surface, cutting off exposure to the air, and thus accelerating the curing process.  That's one of the reasons why sanding and painting help the process.

I added a drop of hardener to some of the patches that were staying sticky, and within a couple of hours, they were pretty much hard (although a little more plastic than I would like).  Also, because I had not put enough resin on top of the portion of the tube where I put in fiberglass cloth, I applied a second coat of resin, using more hardener this time.  This morning, it was much closer to being done than the previous coat had been, and in addition, provides a barrier to the air for the first coat.  My hope is that by this evening everything will have hardened enough for me to start sanding.

The inner part of the tube remains a little more sticky than I would like, but this may be an advantage, actually, because it will make it easier to get flat black paint to stick to it.  There really isn't a pressing need to sand the inner part of the tube; a rough surface reflects less light than a smooth one, and on the inside of the telescope tube, anything that isn't reflecting is good.

UPDATE: My wife moved it into the sunshine about noon, because it was still not completely dry, and in an hour -- rock solid.

It weighs just over 28 pounds -- so I added a bit less than three pounds of resin.  I sanded it down to 320 grit this evening, which probably took it down below 28 pounds again.  There were a couple of air bubbles where the fiberglass cloth had been, and a few spots were a little sticky under the top layer; in general, the surface isn't quite as even as I would like, but that's why you sand and add another layer, to smooth these discrepancies out.  It does not need to be commercial fiberglass quality, but I would like it relatively pretty before I drill the holes and paint it.  So I added one more layer of resin this evening, and I will put it out in the sunlight in the morning to harden up good.

1 comment:

Richard Clark said...

If I remember right, polyester resin comes in two types - one for inner coats that doesn't set up hard, and one for the outer coat that has the wax that you mentioned.

I think you can wrap the object in plastic wrap to keep the air off of the first kind of resin and get it to set.