plastic wrap in scarf joints

[jochiges]

Dear All,

I recently glued up some 1/4" meranti plywood scarf joints with epoxy glue. Between the clamping jig and the boards I used plastic wrap as a barrier so the woods would not stick. Unfortuneately, the saran wrap is now glued and embedded into the joint. In addition to that, the bevel seemed to have slipped slightly, and now I am stuck with a slight ridge/valley on one of the joints (with saran wrap in it). I have sanded off most of the plastic and excess epoxy, but the joint is still quite unsightly and I am worried if the strength of the joint is threatened by the slippage of the boards during clamping. Any suggestions would be greatly appreciated by this novice boatbuilder!
Thanks for your time,
John

[Charlie J]

saw it in half right down the center of the joint, recut the scarfs, then reglue. At this point, while you are making panels, is not the time to start accepting poor joints. The strength will never be there and YOU will know it the whole life of the boat.

Cut your losses and redo the joint. A precept I always work from - If in doubt, do it over

[suedebriar]

This same thing happened to me when I glued up some locust frames. I sat back in the moaning chair (you have a moaning chair don't you?) and reflected on the mater. Im my case the plastic used for a parting barrier creased into the joint. I sanded it back and looked again at how much was incased. I decided that the structure was not effected. The strength was still there but it just looked bad. I ended up doing it over. In your case that would be much more costly. I would be tempted to gringd it away until it was gone and fill it with a graving piece.

[suedebriar]

I just reread the post and I think Charlie is right. Do the joint over.

[NormMessinger]

Whew!

--Norm

[capt jake]

Saran wrap has a nasty habit for this. Use a heavier plastic as a barrier next time. The thin nature of Saran tends to 'draw' it into the joint.
G'luck! smile.gif

[JimD]

yup, redo it! Heavy green garbage type plastic will be more likely to come off in one piece. I usually wrap garbage bag plastic snuggly around suitably sized blocks of wood and tape it on with duct tape so it can't wrinkle or otherwise get stuck in the glue. The plasticized blocks lay on the scarf, the clamps tighten on the blocks. The pieces being scarfed are first clamped or otherwise secured in place so when you clamp preasure on the scarf the boards can't slide off each other.

[WRB]

I always use Waxed paper (the stuff you use in the kitchen) and never have any problems as it is quite thick, and so doesn't get into the joint.
Also, always tack both pieces down at the far ends to stop the seam from sliding.

[capt jake]

Waxed paper (the stuff you use in the kitchen) Then why do they recommend NOT mixing epoxy in a waxed container??? I don't think I would use waxed papaer, but that's just me. smile.gif

[WRB]

Because, the wax would effect the epoxy some. However, the waxed paper only comes in contact with that part of the adhesive that squeazes out of the joint and is going to be planed off. there is no wat that wax from the paper is going to get into the joint...

[NormMessinger]

Mixing epoxy in waxed cups was absoultly forbidden with the epoxy I used for the Long-EZ. So I was surprised and concerned when the cups I ordered from System-Three were waxed. I call them. No problem with System-Three, they said. I'd need the mfgr's assurance before doing so with other brands.

--Norm

[capt jake]

I buy my mixing cups from Albertson's, the medium sized cup out of the deli! About .04 per container!!!!!!!!! WOW you say, so did I!!! Check out the local deli and buy 'em in bulk (12-24 at a time)!
Wow!

[On Vacation]

My amateur brain freeze on this subject. Dry fit the joint and put a pencil mark on the scarf area so you will have a guidline to refit the joint when gluing. Check for true or straight line on the running side of both pieces of plywood before gluing if you intend on using the whole piece in your patterns.It is not a shame to have excess glue or stuck wax paper to joint. Heat from the epoxy kicking will do this many times especially in hotter weather or climate.

A quick fix to stop the slipping is to use sheetrock screws through each side or the plywood to backing boards before clamping after aligning the scarf joint with the glue in it. I would say that the wax paper would only be in the excess glue in the void of the joint and would not effect the strength of the joint.

If you don't get the joint a true uniform thickness and true taper, which is not uncommon, the clamping board on the joint will not always apply even pressure.
Don't apply too much pressure as it will starve the joint. Check the crown on the clamping board to find the inverted bow and put the crown down before clamping. This will apply pressure to the middle after tightening down on the outer clamps.

Also find some deep throat clamps if all possible.

[Pelican]

I use waxed paper with good results. I like the glad containers they make for storage when mixing. More room to spread out so it doesn't cook off as fast and you let any leftover smile.gif set and squeeze. It all pops out and you use over and over and over.

[carlg]

To keep the scarfed pieces from slipping I use a few grains of sand right in the joint. And I do mean FEW. Hardwood 1 x 2's will stay in place with as little as 4 or 5 grains.

[jochiges]

Thanks for commenting on the problem, fellas!

I suppose redoing the joint altogether would be the most secure solution. I've reinspected the joint itself and discovered that the valley i saw was in fact the feather edge itself, which was not straight in the first place (hard to explain, but the edge of the bevel was very fine and therefore uneven at spots). It's still there though, and it does bug me. I was thinking about filling in the small groove with wood flour and epoxy , then reinforcing the whole scarf with fiberglass... The whole outside of the hull will be laminated with 6 oz. 'glass, but if the joint is iffy I would definately redo them.

Thanks,
John

[Ross Faneuf]

It's much easier to fix this joint now, by redoing it, than (possibly) to have to fix the consequences later, when you're installing it or when the boat is in service.

I prevent scarfs from slipping by pegging them with a piece of dowel; you could go as small as 3/32 with 1/4 stock. Lay out the joint dry to check it, drill through in the middle of the joing (through the face of the joint) and have a bit of dowel handy for a pin. Easiest to insert it in the glue face of one side of the joint after slathering the epoxy. Will not be unsightly. Don't use for alignment, though - just to avoid slippage. Eyeball for alignment before final clamping.