Scarfing/Splicing Sheet Goods for Signs - Wide Clamp

[mrdovey]

I'm cutting a 3'x12' sign in Extira (Thanks, Dale, I wouldn't have known about Extira if not for your earlier remarks on the Forum) but could only get 4'x8' sheets. I'm using a stepped scarf joint and needed a clamp that would provide reasonably even pressure across the entire joint and cobbled up this monstrosity using six veneer press screws and some 2x4 and 2x6 stock.

I think the picture probably tells the whole story:


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Morris

[gerald_d]

Hi Morris, did you see this thread (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=6720&post=26228#POST26228) with a link to www.bowclamp.com (http://www.bowclamp.com) ?
(My grandfather would have gone to the scrapyard for a 3' long spring blade.....)

[mrdovey]

Gerald...

I like the leaf spring idea. I think I might like the bowclamp product even better.

However, I want to be able to control the clamping pressure to suit myself - and neither the leaf spring nor the bowclamp let me do that. Clamping pressure at the end of the cauls (only) almost certainly doesn't generate the same pressure across the entire joint.

I feel a lot more comfortable with six pressure adjustment points across the 49" clamp width - and my clamp "faces" are 5-1/2" wide to match the width of the joint.

...Morris

[gerald_d]

Aha, I only spot the six screws now - the handles are neatly in line with the frame of your shop wall!


5.5" did you say....okay, another trip to the scrapyard to get a second spring to put next to the first one.

[carl]

Morris,
There are some people that use old fire hose's for equal pressures, just close off the one side altogether, put some kind of air valve on the other side, set this in your clamps, don't tighen it down all the way, and fill the hose full of air. This will give you a nice even pressure along the whole joint.
Hope this helps,
Carl

[mrdovey]

Carl...

Another great idea! I've been asking myself: "Why didn't I think of that - and my only consolation is that I can re-use the screw clamps to make solar panel glue-up easier.

Friday I'll ask one of the local firemen to be on the lookout for a hose replacement. I should be able to get 11 or 12 four-foot clamps out of an old 50-foot hose.

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...Morris

[joe]

Got stumped on this subject right off, due to the use of Extira as the materials for a 3'X12' panel.
MDO,in my estimations, is the material choice here.

At 12'it's too floppy, heavy, and doesn't like glue very much. We ran glue tests a few months ago and the reslts were that Gorilla Glue and Epoxy worked best, but generally failed due to the flaky surface. While the glue stuck, the surface tore away about 1/4" down.

When painting, we found it needs a stable penetrating surface coat. Because it's water resistant, latex primers didn't do well for us. Our best result was using a thinned down wash of Epoxy. Two coats and your good to go with most any paint.

My two cents.

Joe

[mrdovey]

Joe...

Thanks for the tips! There won't be significant stress on the 6" wide splice.

I was already planning to add stiffners to the back of the panels; and will give serious thought to widening those for more bond area.

Thanks for the epoxy wash coat suggestion. I'll give that a try.

...Morris

[joe]

Morris,

This stuff in 1/2" thickness will really flex. In 3/4" it's very heavy and will be difficult to brace enough to keep the scarf joint from showing.

If your really set on using Extira, I'd probably build a frame first and screw the materials to it. Be sure to pre-drill the holes as Extira will bulge out when screw down.

I really like this material for smaller signs and it carves beautifully. MDO is a much better choice for larger signs.

Joe

[billp]

Guys,
I recently did some signs for CMI which is the company that markets Extira. In fact they were the ones who commissioned "AL" (the 7 foot Extira alligator I showed on this Forum). I've done some smaller projects as well, and the current issues of the major sign magazines have one of my Extira signs featured on their back covers.
What I learned from these projects (and what Extira themselves didn't seem to know) was that after trying most sealers which were suggested, the best option turned out to be good old shellac again ! I put on two coats (one thinned, one straight), and when sanded the surface was glass smooth. At the recent Las Vegas woodworking show I spoke with their reps and they all seemed surprised to hear this, since no one had thought to try it out on their own...I think the alcohol base somehow breaks up a little of that surface "tension/coating"which seems to keep the other primers from adhering properly.
And in going back to the original concept which started this thread (clamping down scarf joints), I'm wondering why some sort of vacuum bag/clamp couldn't be adapted just as easily as weight/pressure from mechanical devices. The bag would have perfectly even pressure over the entire joint, it could probably be cobbled up from plastic tarp and duct tape, and the plastic should NOT stick to the glue...I'd probably want a good filter in the vacuum line however as this stuff is incredibly pervasive.....
If you are going to take on a project using this material I'd suggest charging a premium, as you WILL spend extra time cleaning the formaldahyde dust from everything, and you WILL destroy tooling rapidly, and it IS 15-30% heavier than regular MDF...

[wooddr]

Bill,

I just tried exteria for the first time on some exterior fluted posts I am building for a new patio cover I built at my house. After machining the flutes I was surprised at how rough they were compared to exterior mdf (medix I believe).

Now that I have them cut up and I am stuck with them I am ready to seal them. I have been trying to find a good exterior sealer. I haven't ever used shellac before. Is is a good sealer for exterior use, Or is an exterior grade oil based primer thinned down better?

Thanks

Dirk

[billp]

Dirk,
While shellac has been succesfully used for a few thousand years so far, I'm really not sure where it stands in comparison with other current options. In many cases I use it as a primary sealer, and then I overcoat it with a varnish, water based poly, or an epoxy. It is relatively "soft" when it is cured, if compared to a polyurethane, etc. But it's also held up extremely well on children's furniture/toys for a long time ( and as an aside it is non toxic when cured...).
It can also be an attractive finish as well.
As far as "better" goes when compared to a thinned down oil based primer- I don't know. It would seem to imply that you will be overcoating your primer anyway, so I'd opt for the primer which dried the fastest (shellac is alcohol based so it is dry within a half hour, while an oil based primer might mean an overnight wait...).
If anyone else out there has already run these "tests" please chime in an let all of us know about the results.

[joe]

Bill, and other router heads,

I agree with Bill P. about Shellac as a good penetrating sealer. The primer "Kils" is shellac base and used to trap the odor left behind after an interior house fire. It will also trap oily pine Knots. This is almost a forgoten product.

Still, shellac is a soft sealer. It dried fast and sands beautifully. However sometimes you need a harder surface, hence I use the epxoy wash.

What is Shellac? This is a natural product reduced from the Lac Beetle. Usually from India. Seed lack comes from the Beetle larva. You can find it ready to use, in a 3lb cut, Bulls Eye Shellac, or you can purchase dried shellac buttons. There about the size of a hald dollar, about a quarter inch thick. It comes in several colors, with natural wax or de-waxed. I like the buttons best. You can drop a few in a jar, pour in the alchohol, cover and let set overnight for a fresh solution. The curder, darker orange buttons needs to be strained before use. Getting out the little bug parts.

Mixing fresh guarantees fast drying. It's cheaper too. One other benefit to shellac is safety. It's not only used in our paint products, but also in many food materials as part of the hard coating for phamacuticals and candy.

If interested you can Google Shellac for more information.

When you forget and leave out a shellac brush overnight, and it's hard as a brick, no problem, let it soak in a little alchohol, and it will be good as new.

Thanks Bill P. for the suggestions. I've a schedule conflict with the Beckwith Camp. Will be attending the Sculpt Noveau seminar in NYC at that time. Will see you at the ADA Camp though.

Joe

[mikejohn]

It's been asked before on this forum, but yet to be answered.
What is an epoxy wash?
If I Google 'epoxy' I get over 300 products,glues, sealers,floor coverings, clear casting etc.
What type of product are you using, and how do you make it a 'wash'?
Thanks
.....................Mike

[bleeth]

Joe: Here in South Florida Kilz is probably the largest selling primer going. (Having a lot to do with our constant battle against moisture). For those not familiar with it a reference to the products tab on their home page will lead you to their several formulations including a specific exterior application formula.

http://www.kilz.com/

Dave

[joe]

Mike,

I first heard of making an epoxy wash on this forum. It's nothing more than reducing epoxy with lacquer thinner.

Each brand of epoxy has different viscosity so dilutions is by eye. I dilute by about a third thinner. It's not a critical measurement and will set up in a normal way.

Thanks again to this forum.

[mrdovey]

Early observations on cutting Extira:

As a preliminary to fabricating the sign, I cut one 3/4" sheet into hexagonal (2"/side) game pieces for another customer.

Initially, I cut the hexagons with two 3/8" passes (new 3/8" bit, 1.5"/sec, 18kRPM) and after 54 hexagons, my y-axis movement started wimping out. I stopped the program and shut down to re-assess the project.

It turned out that the 108' was the most I could hope for from a brand new carbide edge. I modified the code to allow a checkpoint restart, chucked up a new bit and cut 100 hexagons with a single pass and paused to check the bit. Sure enough - the bottom 3/4" was noticibly dulled.

Changed the bit and restarted the program - and after cutting another 100 hexagons, the new bit was also dull. Changed bits once again and cut the remainder without problem.

Extira is dusty stuff! As Bill pointed out, it's abrasive as all get out (and I /will/ charge a premium next time!).

And yes, it's heavy stuff. Dale had warned us some time ago that the stuff was heavy - but the tilt-top cart he prodded me to build helped a lot in moving sheets around. I posted a drawing somewhere on the forum (I've forgotten where) and have pix on a web page (http://www.iedu.com/DeSoto/shop_cart.html).

I'll use the epoxy wash as a pre-primer - and may use it to strengthen areas of the reverse where I plan to glue stiffners to minimize flexing.

Thanks to all!

[ron brown]

On Epoxy - somewhere I read this and don't remember where. One can "spritz" a final coating of epoxy with alcohol - and I would think lacquer thinner - and it will lower the surface tension and leave a smooth coat.

From the manufacturors - cutting epoxy with laquer thinner will lower the water-reistance and moisture exclusion qualities of epoxy (duh).

From my shop - after applying layers of glass cloth to a "bright finish" boat hull, it must be filled. After the light sanding to kill the high spots, extreemly light cloth or "peal ply" can be used and either will lower the number of times epoxy must be applied and the amount of sanding.

Shellac can sometimes be found in flakes also. As Joe mentioned, fresh shelllac is best.

Ron

[joe]

Ron,

It's true about a light spritz of denatured alchohol will bring out all the bubbles left behind by a roller, leaving a glass smooth surface.

We do it on all our epoxy coatings.

Shellac comes in more colors and purity in flakes and they disolve better than buttons. Some of the old 78 records were made from Shellac. Violin makers will kill for them. They render such a rich dark brown color. Often found in garage sales.

J.

Joe

[mrdovey]

To follow up, here's a drawing of the simple glue joint and a photo of the clamp in use. First use was with Titebond III, and second was with epoxy. The strange color of the upper clamp face was
the green cling wrap that I used to avoid gluing the panel and clamp together.


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...Morris