Sanding lap joint edges

After months of slow progress, due to room temperature issues, and the general insanity of the world, I have gotten back to building my Skerry. I'm finishing up the gluing and sanding. While epoxy runs are visible, the surface of the hull is pretty smooth.

My questions for today concern the seams between the panels (between panels #1 and #2, and between #2 and #3). I was unable to fill in those seams perfectly smoothly. In some places the epoxy is level with the top of the seam (e.g., where the 2nd panel extends past the 1st), but in other places there is a surplus of epoxy, and in general, it isn't very smooth. (See attached photos. Some were taken before sanding, some after. The surfaces are smooth, I'm wondering about the seams.)

- How smooth does the epoxy need to be here?

- If I need to sand down the surplus epoxy in these seams, what's a good tool to use?

- Do I sand down the #2 lap joint edges where they extend past the #1 panel? I can't really make this out from the pictures in the manual.

13 replies:

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RE: Sanding lap joint edges

I used paintscrapers like this one: and this one: paint scraper / p99303

You can try to heat the epoxy in the seams first with a heat gun.

A the end i used a rabbet plane.
In all cases: be careful!

RE: Sanding lap joint edges

Those lap joints are there for a reason. Sanding the outer plank surfaces so those laps disappear will seriously compromise the strength of your hull.

The idea is to use the gap between the panels at the laps as space to be filled with epoxy, then form a careful fillet of thickened epoxy on top that gently and smoothly fairs into the plank surface above.

If what I see in those pics is what you're confronted by, you'll need to remove as much of the excess epoxy overfilling each lap joint, maybe with a rat-tail file rather than a simple scraper. Scrapers are fine for dealing with drips or high spots but not too efficient for removing a lot of material to form a surface carefully. Rifflers are another similar kind of file-like tool, available in a wide range of sizes & curves.

(Which image-hosting service did you use for your pics? That I can see 'em at all is a big plus! I've had no end of frustration getting images to appear here that others then can see... I have some from my current build I'd post to show how I've finished my laps if I had any confidence others could then see 'em!)

RE: Sanding lap joint edges

If you can view that image you'll get an idea of how the panel lap fillets ended up on my hull. That was taken while my hull was upside down so I could work on the filets w/o the hassle of gravity working against me.

Panel laps are structural; the minor gaps between panels need to be solid-filled with epoxy then finished nicely so that fill blends into the panels beneath them.

Besides being structural (in the sense that they stiffen the edges of the joined panels at the joint interface) they also serve to slightly increase the hull volume as the hull heels in use, creating a bit of additional buoyancy missing in a smooth-hull design.

RE: Sanding lap joint edges

spclark: That's a really nice job. I doubt that I can get there from here, but will look into the tools that you mentioned.

I tried using a variety of small sanding bits (various shapes, sort of a fine stone texture), with my drill, but the epoxy was pretty resistant. That's not working.

About the images: I have the smallest possible AWS instance, and run a web server there. I just put the images on my server.

RE: Sanding lap joint edges

Yeah, tougher row to hoe once the 'pox's cured hard. Best taken care of when it's just getting stiffish, either using a tool or a gloved finger wet with denatured alcohol. Even water if alcohol's not available.

Sanding discs / drums'd take awhile, too little removed before the grit clogs. Carbide burrs work but need real careful use or you can gouge the plywood or a finger, takes practice & patience.

3/8" rat tail file'd be what I'd use first on what your pics show. Take your time to learn how it works, buy a 'file card' to clean the teeth when they get clogged. Wear heavyish gloves or you'll have blisters on your fingers PDQ.

And save your doubts for other uncertainties. You CAN get those seams to look like mine if you follow my instructions & have patience learning how to approach the fix.

It's Not That Hard. Takes time though, and effort.



RE: Sanding lap joint edges

Not a chain saw file either, they're too small & the teeth are too fine. You want one of these:

- and that 'file card's' a brush on a stick used to clean files when their teeth get clogged:

RE: Sanding lap joint edges

Thanks, I'll try that.

RE: Sanding lap joint edges

Actually, about that file you linked to: It says that the diameter is 0.31", and you recommended 3/8" (0.375). Close enough, or should I look for something actually 3/8"?

RE: Sanding lap joint edges

0.31'll work, I just picked that link at random to distinguish the rat tail file type from tye round chain-saw kind that you don't want. Check any well-stocked hardware store, you should find one.

Rat tails are tapered, the pointy end's usually flat-cut while the tang's square, ~ 2-1/2" long. You don't use a handle for the operation you need to do, just hold the file in your fingers, work it along those laps from right to left if your a right-handed bloke. Pointy end to the left, tang to the right. This works best as file teeth cut only one way; back'n'forth motion'll clog it faster.

Coarse sandpaper (80 or 100 grit) 'broken' across a sharp table's edge, so it'll wrap tightly around a dowel, works too. Sandpaper's not as easy to clean and a dowel's not tapered so it's different than using a file but effective if it's all you have.

I think the file's faster, you can use that taper to good effect where your laps grow less deep... it's a 'know your tools' kinda thing which one's preferred.





RE: Sanding lap joint edges

Riffler files -

- come in sets usually, need a little more effort to find locally. They're smaller so it takes more effort to work a large area over, but they can be worked into areas a rat tail file just won't be able to get into... so again, they have an application but wouldn't be my choice unless I didn't have anything else.


RE: Sanding lap joint edges

 geophile -

I started my Skerry in a class at CLC.  It was an intense week.  I am meticulous by nature, and the pace was such that I simply wasn’t able to make any of my fillets as smooth and finished as I would have liked.  Consequently, I came home with a whole lot of cured epoxy still to be sanded off.  I considered using a rat tail file to shape the lap joint fillets, but I ended up using sandpaper wrapped around a round pencil.  The uniform diameter combined with the length of the pencil gave me really consistent form and I felt confident using 80 grit sandpaper without any concern about gouging or removing any of the lap joint edges.  It was tedious, but it came out very nice indeed. 

Judging from your photos, it looks like you may actually want to build up the epoxy a bit in some places before final shaping.  I’m thinking a popsicle stick might make a pretty good filleting tool.

FWIW, I just measured the pencil I used – it was 0.302” in diameter.


RE: Sanding lap joint edges

Agree with hokker abour sandpaper around dowel or round pencil, a practical choice you should try. Renember to 'break' the sandpaper over a square-edge benchtop first though or it's likely not to roll smoothly around your dowel.

Idea here is to grasp two opposite edges with your hands then pull sandpaper between them slowly across a hard, straight edge with your hands pushing lightly towards that edge. This 'breaks' up the coating of glue on the paper and relaxes the paper's fibers so it'll bend evenly, not make hard spots or creases that will gouge your work.

I'd get a start first on evening out those glue joints first before adding more epoxy too. This time use wood fiber to thicken it some; it stays in place better, takes less epoxy and is much easier to sand / file once cured. Only caveat is it's best applied over a thin coat of unthickened epoxy that's begun to set up to the 'tacky but not hard' stage so your filleting mix doesn't slide around too much. Then once it's cured about half-way you can easily push it around with tools or an alcohol-wetted gloved finger to both even out high/low spots as well as give the mix a nice smooth finish that will later requier much less sanding.


RE: Sanding lap joint edges

(DRAT where's that edit button when you need it??)

"Breaking" sandpaper - maybe it's obvious, but if not: when doing this maneuver, it's the paper back side of your sheet that gets broken across a table edge, NOT the abrasive side....

And doing it both ways (turn paper 90°, do the other axis the same) is good if you want to use the paper around a soft foam block or folded up cloth for sanding non-flat surfaces.

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