Wednesday, August 12, 2015

Fiberglassing in the Dark

It is kind of funny.  It seems most projects that my wife and I start, at some point along the way, result in performing work late at night in the dark. Be it planting trees in our old backyard, re-tiling our old bathroom, or a myriad of other projects, we end up misjudging the amount of time needed.  And now we can add fiberglass layup to the list.

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 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 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.


  1. Mike - the purpose for the regularly patterned holes is so that when you press/weight/vacuum bag the core into its catalyzed substrate, the air beneath the core will get pushed/pulled out the holes so you don't get big air voids beneath the core. I used to work for DIAB as an engineer. You'd be surprised how well the vacuum on an older pickup truck works to hold a vacuum bag when the power goes out. Or, how well a shop vac works in the backwoods of Alabama when showing a crew in a shop with a dirt floor how to bag large panels. Oh, the stories .....

    1. Hi Dave. Well, I guess it would have the same effect on the first hand layup on one side (providing you can squeeze the bubbles over to the holes). So, in my case, doing hand layup, is it worth the effort to poke all those holes (it is a time consuming PITA)?

    2. Do your first layup down hand. Do you have enough perimeter to add some butyl tacky tape? You need at least a couple inches - it really is not that difficult. Plastic sheet from Lowes/Home Depot works fine, you aren't doing aerospace. And a shop vac. You may not have noticed a difference but down the road, if any water, more air, laminate separation starts to happen, well, you could get a massive separation of the sandwich panel.
      Go to youtube - there are a million examples. Just sayin'

    3. Hey Dave, I'd love to try vacuum bagging but I just don't see how to do it given my setup. I don't have any perimeter outside the layup where I could seal anything. I've got several flat sheets of 1" foam and a curved set of stringers that I need to bend the foam over. Since it isn't a solid mold (and I have no real way I can think of to make a curved table that I could seal against here) and best option I seem to have is to tie the foam to the curved stringers, I just don't know how I can keep a vacuum that wouldn't draw in air and suck out the resin.

      A couple general questions for you though. If you are doing a large part and vacuum bagging it, how do you get everything assembled before the polyester starts to cure? In the temperatures here, I just can't see getting a mix that will last long enough to get everything laid up and bagged before it cured. And for the shop vac approach, how do you attach it to the bag without the usual fittings? And to get the vacuum, don't you need other material in between the part and the plastic that allows the vacuum to pull out all the air? I thought the procedure was to use peel ply and some sort of media/fabric that will allow the pump to suck the air out? And if the bag is used, then won't the laminating resin fully cure?

      Even if I can't do it this time, curious to know how your low-tech process would work.

    4. Hi Mike -
      Been out sailing. Envious? :-)
      With a vac bag, you get all your tacky tape in place around the perimeter, leaving the tape upper surface papered on three sides. The fourth side will already have the plastic sheet stuck to it, with the sheet folded away from the part you are building. You can catalyze at the low end 1-1/4% or so or use a BPO activated putty that has more leeway for adjusting catalyst times. It helps to do a test sample off to the side to be sure of time. Yes, once you put the matrix down, you need to move fast (that's what's so nice about epoxy and longer pot life). I have also refrigerated the matrix to get a cool start on it.
      For the vac penetration, cut a hole barely large enough for the vac hose fitting, stick the hose in, and seal with tacky tape all around.
      As far as something to spread the vacuum, bubble wrap under the plastic works great (keeping it in from the edges a bit), or you can take poly tube, drill holes in it and snake it back and forth on the core surface, holding in place with tape or tacky tape. The vac has to be located over the tubing in order to work.
      The peel ply makes it easy to remove everything and leaves a fairly nice surface, but it is not necessary. The plastic stuff will all peel away and the surface might be a bit lumpy, but as you found out, it needs to be sanded smooth anyhow.
      The cure will not be inhibited under the bag, The process increases the odds of getting a uniformly wetted laminate. Some cores will off-gas (don't believe those who say Divinycel does - it's usually process caused in this case) and some cores, with a slow gel time resin, will react negatively to high styrene levels. It's always good to test your process and chemicals if treading into unknown waters.
      I would imagine there are a bazillion videos on-line showing everything from basic to advanced.
      Looking forward to seeing your completed product.

    5. Hey Dave,

      Thanks for the info. Nice to know the cheaper substitutions for some of the expensive vacuum bagging stuff. I wish I could vacuum bag the top, but I just can't see how to do it with my current setup. Right now I'm just trying to keep it from raining on what I have done. Working out in a boatyard is far from the ideal anyway.

      And yes, I'm a bit jealous. Would much rather be having some fun than doing this...but this is a step toward that goal anyway.

      Did you ever find a light for your instrument cluster? Seems like a LED would be better than any sort of incandescent bulb.


  2. Hi Mike,
    Would a 1/4" -1/3" deep larger dia nail hole, a dimple or shallow 1/4"-1/3" V (none going all the way through) into the top and bottom of the foam increase the surface area and mechancial bond of the resin to the foam be a substitute to nail holes all the way thru and having to sand hundreds of drip thru's? Would need to be shape that allows flowing of resin into hole without a bubble on top- assumes you put resin on and do layup while mold/foam is nearly horizontal.
    Shop vac is a possibility if you have, small Vacuum pumps run around $100 at your favorite Harbor Freight ( but I doubt you would have a future use for), but then you need the sheet plastic, tape/hoses etc
    Mostly it's a matter of building a strong enough bimini for your needs, and it would be easier if you don't have to vac-bag.
    Doug from VT

    1. In my test panel I only did half of it with the holes. Honestly, I didn't notice any difference in the panel. I've even jumped up and down on it while it was suspended by the very edge of a couple of wood blocks (about 1" x 4" lip). So, I think the hand layup will work fine. Having said all of that, sanding the nibs off the back wasn't that bad...came off with 60 grit pretty quickly. The idea of them releasing any trapped air seems to me to be the bigger possible advantage if there is any air trapped under the thick mat.n The bigger pain is pressing, and then removing, the board full of nails. Even my 1 foot by 2 foot nail board is difficult to pry off the foam once it is pressed into it.

  3. Mike, you can purchase a "texturing roller" from Jamestown. It is used to apply a non-skid coating called Kiwi Grip (I think). I have used it on wing walk on airplanes with good success.

    1. Thanks guys. I did find a roller that I think will work when using thickened gel coat. Picture of my various trials posted on my Facebook page: