I got some concerned messages about our pre-coating strategy in the Blade building.
Basically what we did was put on a thin layer of epoxy over the wood and let it harden overnight before putting on the glass. Idea is to stop the wood from gobbling up too much epoxy when glassing which adds weight but little strength to the hull. We sanded and washed the surface before we put on the glass.
Since people are concerned, I tought it might be interesting to do a peel test on the glass. We have not cut the stem to length, so we can do the test there. How hard should it be to pull the glass off the wood when the epoxy has cured? Is it good enough if it pulls pieces of the wood off? <img src=
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Opinions on how to do the test? We have about 3-4cm of width to work on. How many kilos of pull should be neccesary to pull the glass off 90deg to the wood..
I hope you are not doing this pull test on the actual hull. Why not make a test panel ? Is it safe to mess with so much work done ? Sorry no advice to give, but still, a test panel might be better.
When we cut off the overlength stem, we will have a test panel of 3-4 cm width of the length from the deck to the keel. Gives us the opportunity to test the real thing.
Rolf, I assume the peel strenght should be close to the numbers specified in the resin datasheet. The datasheet should provide both chemical and mechanical bond peel strenght. To measure force you can probably use 5litre pet botles filled with water( hanging from a wire clamped to the glass) , and keep adding them until the glass peels. Keep us posted.
You must absolutely remove any surface coating on the
old
finish ie.
your wet out coating of the wood. Sanding with 80 grit paper should to the trick. The problem is that it easy to be a little to enthusiastic and sand to much.
In the future I would use peelply on the surface after you put the epoxy on it. This will soak up excess resen and when it is pulled off it leaves a coarse
tooth
for the next layer to bond to.
A note on fiberglass, you should use the finish (on the glass) that is designed for your resin. Silane finish for epoxy resins.
Yes, sanding and drying off with a clean moist towel. We want a good grip for the second layer so we are following procedure.
I tought it was common practice to always sand now, even after using peel ply. Wasn't it discovered that peel ply wasn not sufficient to give a reliable secondary bonding in the experimental aircraft hobby?
We are definately using glass intended for use with epoxy as we are probably ending up with a clear finish to begin with.
Any toughts about the peel test?
Either the timber should pull up or the fibre will break where it is held. Testing near the edge of a laminate will be tricky because it is usually a resin rich or resin poor area. Otherwise, cut one skin then
crack
the timber to leave just the other skin holding the two parts together. Then try to rip the glass off one part at 90 degrees, one part cantilevered off a bench, the other dangling off the side with increasing weight added to it. There should be an engineering standard around for an alternative test.
Peel plied resin requires a quick sand to get good adhesion with successive laminates, but not as much as non-peel plied resin.
I would personally avoid using water or other solvents to wash an area to be laminated. An air blow-down and/or a good brushing and/or a vacuuming should be enough, and doesn't take very long.
Why didn't you just resin the timber, leave for 30mins or so before getting too tacky, then apply the laminate. No sanding to bother with and a good chemical bond.
We did some tests with the cutoffs from the stern, and the glass either broke or it pulled the wood off. An interesting observation was that we had to take care not to damage the fibers where we were pulling. A small
kink
or fold there, and wahoo, it broke there instead.
Why didn't you just resin the timber, leave for 30mins or so before getting too tacky, then apply the laminate. No sanding to bother with and a good chemical bond.
I have a compressor but I dont know how good the water separator in it is. Using compressed air would make my garage an even dustier place. Brushing is really not enough, there will always be some dust left. I have not tried vacuming, since I am certain my wife would murder me <img src=
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Wiping the sanded surface with a moist and hot towel (not wet) have worked OK til now. Better options are definatly wanted!
Waiting 30 minutes and then laminate.. Interesting option. We have been working in kind of cold conditions until now so I 30 minutes might be a bit too short for the epoxy to kick or geel. I assume that was your idea, to let the epoxy kick and gel so it dont soak deeper into the wood before we begin to put the glass on? If we have the time for it, we will have a go at that.
Don’t rely see the necessity to let a layer of epoxy to cure before you put the laminate.
Normally I put on a layer of epoxy, then the glass and then I let it for a wile to let the wood and glass soak up the epoxy. After that I remove the air bubbles and add some epoxy to the dry spots.
When the glass has set but still sticky I give a new layer of resin, and that’s it. Alternatively do all at once and put a peel ply last. There is of course a risk to get dry spots, but it’s very small. But before you do the laminate make the surface perfect, no use to let fairing to after laminating. After that you are ready to sand and paint.
The idea with the pre-coat is to apply a very thin layer of epoxy to act as a barrier coat when putting the glass on. Supposedly there are some weight (and thus money) to save that way. Ref: Bjorn Thomasson. Unless you allow that
barrier
coat to at least gel, dry wood can/will soak up more epoxy than you really need.
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Wiping the sanded surface with a moist and hot towel (not wet) have worked OK til now. Better options are definatly wanted!
I'm currently using a refrigerator compressor for vacuuming, so if you don't need too much pressure that is really cheap silent alternative to vacuum pumps i.e. get it from recycling center or such. You could use also your compressor with ejector (?) but that generates so much noise that I needed to find another alternatives <img src=
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Also I guess that you'll need to get water separator anyway if you are going to spray paint your hulls.
You are maybe right about that. Normally when hand laminating on wood I arrive to 800grams epoxy for 1kg of glass. It would be funny to check the difference if you pre coat.
There is another thing that in my opinion can save a lot more weight had the case on the DS12. One hull was made from ply stored for a year in dry conditions, and the second that came straight from the supplier (stored in a cold store), and I have a difference in weight of 800 grams. As I make my epoxy by weight not volume I normally note everything that I use and there is no difference of any importance there.
Maybe we should start a new weight watchers thread.
Dry wood should absorb more resin than less dry wood, but is it ok to use undried wood, even if it may soak less resin ?
Nope, it’s not at all advisable to use wood that it’s not absolutely dry. The faster after the wood is coming out of the drying kiln the better. Storing wood in a cold store or semi warm will raise its humidity quite fast as we normally have a relative humidity 70-90% outside at least in Finland. For the DS I wanted to go fast and it is a proto not supposed to last very long.
Gato, can you define dry? Wood will never be, and should not be, 100% without water as far as I know.
About 12% a big max 15%,it's still more important if you use ply.
Either way is suitable, all that needs to be done is make sure the glass fibres aren't dry. Don't rush off to grab a beer too quickly.
There is a problem with the method I described and that is having the initial coat being too sticky and not being able to smooth out the dry glass. Applying pre wet-out glass may overcome this.
The same is the case with foam, it soaks up a lot of resin. Apply a
screed
of sloppy/thin bog to the foam before laminating. This will fill the holes and prevent too much heavy resin from soaking in while providing a reasonabe bond for the skins. I don't think this method will work with timber because there are no big holes for the bog to be worked into.
This does depend on the consistency of the mixed epoxy being used and the type of timber. Thinner epoxy will soak into porous timber very quickly, to the point that it looks and feels dry after soaking in for 15 mins. Thick epoxy can leave a glossy painted finish to dense timber.