View Full Version : Epoxy coverage for laminating doug fir

Peter Vella
04-15-2013, 07:07 AM
I am going to be ordering materials for the laminated stem knee on my cutter build (http://forum.woodenboat.com/showthread.php?161699-32ft-cutter-build-in-Wales). I am going to be using doug fir and I have figured out that the total glued area if I am using 1/2" lams is in the region of 8 sq m (86 sq ft).

I have looked all over the West System website and can't find any useful information on coverage/quantities except for with laying glass. Does anyone know from experience roughly how much epoxy per sq m/sq ft is needed for a strong laminated structure? Also some sites recommend microfibres to thicken the epoxy when laminating. Is this necesary or will unthickened epoxy work if the timber is clean sawn or planed?

wizbang 13
04-15-2013, 07:30 AM
If I were to try to figure it out, I might go by the figures for glassing.
miles of laminating I have done , I never have worked with numbers like that .
But , I have learned that the best amount is to use a LOT!! .
Thickened ( with microfibers) uses more than un thickened, = good
Sawn , and even additionally roughed up with a sawzall blade uses more epoxy= good.
You maybe really should not be having this laminated piece your first bit of epoxy work?
Maybe build a few big curved laminations different ways , wait a month , and go smash em against a tree? order a few gallons for these tests.

04-15-2013, 07:50 AM
I can not give guidence on coverage. Wet both surfaces out with unthickened epoxy, then apply thickened epoxy to one surface and clamp. If you did not get squeez out along all outside edges you did not use enough. My guess is that coverage would be half that of wetting out cloth. System three has a user guide on their web site, they may information. Clean up as much of the squeez out as you can so there is less sanding to do latter. I hope this is of some help.

Peter Vella
04-15-2013, 08:50 AM
wiz - there is alot of glue area for a piece this size if I am using 1/2" lams. I could go thicker but I want to minimise springback and thicker lams mean more clamping pressure needed to hold it to the form, so surely that's not good for epoxy? The tightest curve is a radius of roughy 97cm or just over a yard.

As long as I use enough glue and I don't over clamp, just use enough pressure to bend the lams into the form, I don't think I have to worry too much about strength of the bond. I will be working on it in a heated workshop so no problems getting a good temperature for curing.

sean - What if there are close tolerances between the lams? What you describe sounds like you need a layer of unthickened epoxy to penetrate or 'prime' the surface and then the thickened epoxy to bond to the first layer and fill up voids between the lams. If the surfaces being glued are a good fit to start with then wouldn't thickened epoxy and too much clamping pressure lead to a glue starved joint?

04-15-2013, 09:20 AM
I would dry bend all your lams first and let em stay like that for a few days. Every bit of wood is going to absorb different amounts of epoxy, so its difficult to say exactly how much your are going to use. Generally an un-thickened primer coat is applied first so that the epoxy is not sucked into the timber leaving a joint full of micro fibres and no resin if you just went for the thickned brew first.You do want squeeze out, but you dont want a glue starved joint. If you are in a position to have good tolerences, clamping pressure and heat, you could use resourcinal glue.

Peter Vella
04-15-2013, 10:14 AM
skaraborgcraft - The doug fir will be sawn but I need to resaw and thickness it, so it will end up planed, which is good for resourcinol and I would think ok as well for slightly thickened epoxy so long as the surface is keyed like what wiz suggests. I am reading up more about it on West's website now.

I have considered using Polyproof resourcinol. This thing that puts me off is achieving high clamping pressures evenly throughout the whole of the lamination. From my experience with veneer work using cauls and clamps I know that it is hard to apply even pressure reliably without vacuum bagging, especially around curved surfaces.

Another thing that puts me off is high temps. According to the datasheet "Constructions with tensions and hardwoods requires at least 60C":


Epoxy seems so much more forgiving which is probably why it is popular and easily available, wheras resourcinol is hard to find for sale in DIY quantities.

David G
04-15-2013, 10:15 AM
You could call West's toll-free tech support line and ask them.

04-15-2013, 10:29 AM
I did lay up an oak stem for a Waterwitch which was glued with polyurethane. Those lams were not more than 1/4 in. I know a lot of people here dont like it as a glue, but i speak from experience, that i have never had issues with it. It was backed up with bolts though. Sometimes you need a glue that will work in damp and cold +5 tempertures. Not all boat work can be conducted inside a heated shed.

04-15-2013, 10:46 AM
I think you make a good choice when deciding to go with epoxy. You may still need lots of clamps, just keep the pressure as low as possible you will be fine...

http://i1087.photobucket.com/albums/j471/Ekstrom73/IMG_4394.jpg (http://s1087.photobucket.com/user/Ekstrom73/media/IMG_4394.jpg.html)


Gib Etheridge
04-15-2013, 11:09 AM
86 sq. ft. sounds like a math mistake to me. That would be more than 8' x 10' all spread out. That knee doesn't look that big.

If you lay a layer of 6 oz. cloth between each layer you won't have to worry about glue starvation. The cloth, as thin as it is, will hold some glue despite the pressure. This will also stop it from ever splitting.

You can lessen the pressure by dry bending as Skar says, or by steam bending (take it off of the form and allow it to dry thoroughly before glueing), or by laying up only 1/3 of the lams at a time and overbending a bit.

I would buy a gallon of WEST with pumps. It will keep forever, and you said something about laminating deck beams as well.

04-15-2013, 11:34 AM
Peter just thicken the epoxy enough to hold on a vertical surface in a thin layer without slumping. It does not take much pressure to get the squeeze out. the amount of pressure in the middle is likely to be high. This is more a factor of the pressure needed to bend the lams in. If you can work cold to give yourself more work time and then cure with heat. Doing a practice run is good idea. It will let you know your work times. Give yourself enough time to recover from something going wrong

Peter Vella
04-15-2013, 12:17 PM
Robbins sell West epoxy in 6kg and 30kg packs. I think one 6kg pack would cover this job. It might be worth buying two 6kgs considering that it will keep for a while and I will be using it for other jobs later on.

Pre-bending the laminates is good. Another thing to prevent too much clamping pressure would be to apply pressure on the end of the lams with a bending strap to get them into shape, that way there will be less pressure needed perpendicular to the joint to pull them onto the form, instead just enough to hold them all together. This will require the lams to be cut accurately to length. The lengths can be worked out duing the dry pre-bending stage.

Gib - there is alot of glue area, something like 18 * 1/2" laminates at 200mm (8") width and 2.7m (~9') length for the inside of the knee and another six shorter ones for the outer portion plus some blocking to make up the last bit. I will review my maths before ordering materials though.

Bill Perkins
04-15-2013, 01:30 PM
Peter I laminated a 7 ft. stem out of Doug Fir that has proven to be sound . Made a number of other laminations with the same material . My approach to keeping the clamp pressure as low as possible was to saw thin laminates . This also eliminated spring back .My lams planed down to about 3/16ths in. by 3 1/2 in . A Min. radius of "just over a yard" is about what I had on my stem.

My rule of thumb is to find the thickest lamination dimension of the material that will fit on the form without breaking . I then make the laminates for glueing half that thickness ,as a first try .I clamp them dry in the form to pre-bend them as mentioned .This also tells you whether you can bend the mass single-handed for the glue up or not ( clear Fir is stiff ). The thick piece becomes a caul placed on top of the work to even out the clamp pressure .I used System 3's "T88" : a thick clear epoxy that typically needs no additives or pre-wetting .Just coat all surfaces liberally .

Gib Etheridge
04-15-2013, 03:12 PM
I too think you will have an easier time of it with 1/4" lams.

04-15-2013, 07:48 PM
Depending on how rough the surfaces of your fir are; you will use about 1 ounce of mixed, thickened epoxy per square foot. That would be spreading on both sides of each lamination, except the first and last of course. If the wood is smooth, and you spread the epoxy to a thin coat; you will use a little less than 1 oz per sq ft.

04-15-2013, 08:13 PM
The Gougeon book (p. 53) says 13 ft^2/lb. of epoxy, with 20% waste.