After getting the mold built, I wanted to do a test fiberglass layup. I've never created a large composite (foam cored) panel and have never used polyester resin, only epoxy. I've read about as much as I can on the subject and feel comfortable with performing much of the task, but I think it is prudent to try out several techniques on a small sample before I go about making a 9 foot by 12 foot mess with $2000 (U.S.) worth of materials. This way I can practice as well as try out a few different construction ideas.
I started my test piece by cutting a one foot section off one of the 4 foot by 8 foot, 5 pound, 1 inch thick Divinycell panels. The stuff cuts with a razor knife, but at an inch thick, it isn't a single pass cut. Oh, and cutting the stuff kinda sounds like running fingernails down a chalk board to boot. I then cut my 1 foot chunk in half, so my test panel will be a 2 foot square and I can practice gluing two pieces of foam together and testing the joint.
I think it was someone who commented on a prior post on the blog who suggested I poke small holes through the panels. I think the theory is that this will help with bonding to the foam and, since that is critical in a sandwich structure, I figured I would give it a try. So, using a scrap piece of plywood (I seem to have several to choose from) and some finishing nails, I created a board with nails on a 2 inch grid. I then used the board to poke holes into one of the two panels.
I used 30 pound test fishing line to tie the foam panels to the mold. This would allow them to slide just a bit along the frame and still hold the curve. Using some fumed silica, I thickened some polyester resin (to a peanut butter consistency) so it would stay put on a vertical surface. I applied it to one edge of a panel and then the other panel was pressed up against it. Once that cured I had my 2 foot square test panel of foam.
Next was the fiberglass layup itself. To start, I mixed up some resin and applied a thin coat to the foam surface.The main goal of this step is to limit the amount of resin uptake when applying the fiberglass mat. I'm told if this is not done, the foam might suck up too much resin when the glass is applied and leave the glass resin starved (not good). After letting that initial coat gel, I applied and wet out the first layer of glass. The 17 ounce biaxial cloth with the 3/4 ounce chopped strand mat backing is thick stuff and was a bit difficult to wet out. To resolve this issue, in subsequent layers I've put down a bed of resin and then worked the cloth into it, adding resin to complete the wet out as needed, and this seems to work better. Since I was using laminating resin and wanted to test the bonding characteristics overnight, the second layer was applied the following morning. Yeah, that's the story...it had nothing to do with the fact we finished the first layup at 9:30 P.M. using a crappy LED flashlight we happened to have in the car. By the way, I don't recommend doing fiberglass in a tent in a boatyard at night.
The first layer wasn't my best work, but it wasn't too bad given the conditions. I did a little destructive testing to confirm that there was a good bond with the foam and there was. I took this chance to round the corner on part of one edge so I could try molding the mounting flange. I then thickened some resin a bit and coated the mounting flange on the mold to hold the cloth for the next layup. After mixing up some more resin, I applied the second layer of fabric. Since it was daylight and relatively cool that day, the layup went well.
|Second layer of fiberglass and practice with the mounting flange.|
After the top side cured, it was time to break the test piece from the mold, flip it over, and apply glass to the other side. The part came free of the mold (which I had previously covered with cellophane tape so the panel wouldn't stick) after cutting the fishing line holding it down to the frame and giving it a little push away from the flange part of the mold. Flipping it over, it became pretty obvious what the next step was. Resin had indeed seeped through the holes that were made in the core material as well as drips that ran down the edges, so they had to be sanded until the foam surface was back to flat.
|Sanding the resin nubs off the bottom side.|
Two more layers of resin and cloth and the test panel was complete...at least structurally. Being a somewhat impatient person, I didn't want to wait until I had the finish applied to test the structure. If you recall, I'm using a laminating resin so the surface doesn't fully cure (to allow for more layup layers or better bonding of the gelcoat), so I temporarily wrapped the panel in plastic and suspended it between two wood blocks. With only an inch of the ends of the panel on the blocks, I hopped up on the panel. It supported my weight without any problem. I bounced up and down on it and didn't even see it flex. Whew...glad to know the plan is working and the less-than-perfect test sample was performing well.
|The composite panel seems strong.|
After playing with the panel for a bit, the next step was to apply some gelcoat. I wouldn't normally worry about this, but one of the things we are trying to figure out is an easy way to make a nonskid coating in the gelcoat. I had read that a few boat builders wait for the gelcoat to start to gel and then run rollers over it. The roller supposedly will pull small peaks up in the sticky gelcoat and cure with a rougher surface. We tried that technique as well as using a brush to stipple the gelcoat as it was starting to cure. We found that the brush seemed to work better...but rain was threatening so we had to close up the tent and will check on the success or failure in the morning. I guess the best part of the storm now raging outside is that it is giving me a chance to update the blog...if the internet at the marina will cooperate.
So work continues, and thus far the tests seem to be going well. Still a bit nervous about how I'm going to apply these techniques to a 9 foot by 12 foot panel though.