plywood scarfs

[S. Russell]

I'm scarfing plywood for the first time and wonder if someone can answer a question. I have quarter inch, five-ply and am cutting a two inch scarf with a plane. I have it down to the lower surface ply, which is now about half its original thickness. This is not a knife edge but something like a sixty-fourth of an inch, I would guess. I'm afraid of the edge becoming too thin and fragile. Do I need to get this leading edge down to knife thickness?

[kria]

The 1st and only time I scarfed any ply I just kept on til I had a Knife edge (though I didn't think about, ignorence is bliss). I think once they are glued together all will be fine.

Cheers

Kria

[NormMessinger]

Well, if you don't cut it to a knife edge there will be a little joggle in the scarf or you will not get a complete overlap of the scarf. Go ahead and try for the knife edge. If it gets knicked a little (it isn't all that fragile) the epoxy will take care of it and it will be less noticable than a 1/64" deep valley across your panel.

--Norm

[S. Russell]

Thanks for the response. I'll take your advice and try to get it down to a knife edge. I was hoping I could fill or sand out any valleys or ridges that might be left, but I'll sharpen my plane and have a last go at it! One of the problems with doing a project like this for the first time is it's difficult to judge the standards one should work to. It's easy to spend a lot of time on something and then realize after the fact that you were being absurdly careful, or that what you were doing wasn't going to show in the end product anyway.

Thanks again,
Sean Russell

[Tom Dugan]

One last word of advice (if you're still here):

Make sure the plywood is completely backed up right up to the edge. In other words, lay the plywood on a solid smooth surface that completely supports the plywood's edge. Not only will it prevent the back of the plywood from blowing out, but the extra support lets your plane take whisker-thin shavings as you sneak up on the perfect scarf.

Well, it works for me.
-T

[Bruce Hooke]

What works for Tom also works for me smile.gif I was about recommend the same thing and then I saw that he had already done so.

One other tip -- let me see if I can explain this clearly: if your scarf is 2" wide then you presumably laid it out by marking a line 2" back from the edge of the plywood. When you glue up the scarf you want to make sure that these two lines, on the two pieces of plywood, are actually 2" apart. This is easy to do if you carry one of these lines around to the 'back' of the piece of plywood so that you can measure between the lines as you are preparing to clamp everything down. It is very easy (and tempting) to slide the pieces of plywood closer together than they should be but if you do this one piece of plywood will end up being a little higher than the other piece and fixing this will take a lot of sanding and filling.

By the way, unless you are planning to varnish the plywood it is not really a problem if you get some chipping out along the feather edge. You are better off going a little too far and having some chip-outs, than going not far enough and having a step in the joint. Chip-outs can be filled easily with epoxy but a step will need a lot of sanding and planning to get rid of. If you are varnishing the plywood then you will need to aim for as close to perfection as you can get as far as the feather edge goes -- make sure your plane is sharp and make sure the edge of the plywood is well supported!

[S. Russell]

Thanks for the response. I screwed three eight foot-two-by-fours between my sawhorses and nailed a sheet of OSB over that before I laid the plywood sheets down, so there's lots of support (though the plywood is wider than eight feet - I had to add a support to one side). I was losing my nerve as I saw the leading edges get thinner and thinner. I plan to paint the boat inside and out so I'll try not worry about a little chipping along the edge.

I was quite intimidated by this before I started, but it's actually gone quickly and easily.

Thanks to everyone! I might be back for more advice before this project is over.
Sean

[Tom Lathrop]

Sean,

All good advice but I don't consider OSB as a very good support for the feather edge. The OSB I'm familiar with has a fairly uneven surface. I use a planed and jointed edged 2x6 across the scarfing line and replane the surface if it gets nicked too badly.

I do know people who do not go all the way to a feather edge but it seems like more work to get a level joint to me. Tom Hill makes his lapstrakes that way.

[JimD]

Go for the knife edge, definetly. I agree that OSB isn't the best stuff for backing, but if its new with no proud spots from water, etc it might be only a hair splitting difference. Are you fiberglassing and painting over it? The only time you need to get seriously obsessive about a perfectly straight scarf is if its brightwork and will show on the finished boat. Otherwise, just get it smooth to the point that a ridge won't show through the fiberglass. It won't matter if the scarf is a little wavy or you have to do a bit of fairing with microsphere thickend epoxy afterward

[S. Russell]

The OSB is in good shape, but Tom's right it isn't perfectly smooth. Unfortunately the stack of four sheets is nailed down onto the frame I made and will be a real pain to move and then realign. I'll have to consider this. I'm hoping to get a chance to finish the job up tonight. I do plan to paint rather than varnish, so I hope that will give me some small latitude.

One of the things that surprised me was that this plywood (okoume - if that's how it's spelled) did not come with a factory edge. If you want a sharp, straight edge to work with you have to make it yourself. At the price you'd think it would come with a factory edge. I assume they make it slightly oversize and without the factory edge to allow for a little damage in shipping?

Thanks again,
Sean

[Tom Lathrop]

I've seen and heard about this edge bit from a number of people lately. It's got nothing to do with anything except saving the manufacturer money at your expense. Even cheap plywood used to have good edges. Joining the plywood is a critical place where you need a straight edge. If the parts being cut are small enough, you can establish a straight edge to lay out from after gluing up but it is sure nice to have a good edge to start with.

Over the last fifty years that I have been using it, plywood has gone through just about every stage of quality reduction available.

[S. Russell]

The OSB has perfectly straight edges and ninety degree corners. The five-ply marine plywood has ragged edges, and when you lay the sheets on top of each other you realize they aren't square. It's not something I've seen in any of the books or articles I've read on the subject - you should start by making two adjoining sides of the plywood straight and square. It will make all subsequent operations that much easier.

That's a nice looking boat Tom D. included the photo of.

Sean

[wolfietuk]

There are pros and cons to the manfacturer doing finished edges. much of this ply is shipped overseas. The chance for damage is greatly increased the more it is shipped. The birch ply I get for cabinet work comes with a lot of blemishes on the edges (more every year). Also that little extra (I had about 2 inches per sheet) makes it easy to scarf two sheets together and get a full 16' out of two supposedly 8' sheets.

Rick