Seaming plywood on a painted teardrop

[Salivanto]

My current thinking (subject to change with new information) is that I want to paint my teardrop -- and from that I've determined that I can use 4x8 plywood with a seam. (The walls are going to be 4x10.) What's the best way to make the seam? Can that just be filled with putty, or should I cover the surface with something?

[Arne]

well, first, the seam has to be on a joist/stud. second, it has to be well attached. Third, the end grain is the big weakness. The end grain is what will fail first, 95% of the time.

So, seal the end grain before putting the ply on. Be sure it is well attached to the joist ( I would recommend no more than 3 inches between screws). Don't spare the adhesive.... lots of adhesive. You don't want the seam to be able to flex at all..... autobody filler works very well...

I might also consider fiberglass tape over the seam, probably 3 inches wide.

Some migh use a baten instead, but if the seam is sealed and rock solid, I think glass tape is enough.....

Then a good coating, and I don't think latex is a good coating for a tear, but that is just my opinion......

[Brad Lustig]

I would probably use either Bondo or the two part wood filler that's available now. Another way to join them would be to scarf them together. Scarfing plywood can be pretty tough and you still have a seam, but the two pieces of ply won't separate over time and possibly show the joint.

[Arne]

Test Img

[Boodro]

Hi Sali , I am just about finished with my TD , its 4x4x12 & I had to make a seam. I did a butt joint & glued & screwed ,then after the glue set I filled the seam with Elmers wood putty , PRESSED into the seam ,let dry then sanded. I did not know about scarfing at the time I did the walls, if I did I would have scarfed ! I think the joint is very string & as far as I can tell no flexing, I did have a stud on the inside of the wall where my bulkhead was , so my screws had something to hit. I did not paint mine , but if you sand enough after filling it should not be an issue. Good luck!!

[Brad Lustig]

Minwax makes the 2-part epoxy wood filler that I mentioned. Easy to use, sand, etc.

"The Kayak Shop" has some good information on scarfing plywood joints. I'm sure you could find the same information in other places, too.

[Brad Lustig]

Also, you need to be really careful when sanding plywood. Some plywoods have very thin face veneers and if you sand through them, it's really hard to cover up. Other types of plywood will really show off the grain if sanded too much.

[Guest]

Scarf joints work very well and are strong.
I've used them on the canoe project.
I just recently epoxied three sheets of 5' x 5' together for my full size profile template. I didn't take the time to scarf the joints... But I did go with a spline joint at the seams. I didn't even have to clamp the joints, just shoved them together...
I used unthickened epoxy to saturated the wood grain, then before the mix kicked, I added micofibers to thicken the epoxy and gave the joints a second coat, let it set flat overnight and had a 15' x 5' piece of ply the next morning.
I used Andrew's method of locating the loci points of the ellipses and it worked very well laying out the full size template.
My string did seem to have two different stretching point's on the long upper rear ellipse, but I used a fairing batten to fair all the curves afterwards. I also faired a curve into the 24" flat area on the roof line, between the front and rear ellipses. Hade my profile about 3/8" taller.

[Dee Bee]

well, first, the seam has to be on a joist/stud. second, it has to be well attached. Third, the end grain is the big weakness. The end grain is what will fail first, 95% of the time.

So, seal the end grain before putting the ply on. Be sure it is well attached to the joist ( I would recommend no more than 3 inches between screws). Don't spare the adhesive.... lots of adhesive. You don't want the seam to be able to flex at all..... autobody filler works very well...

I might also consider fiberglass tape over the seam, probably 3 inches wide.

Some migh use a baten instead, but if the seam is sealed and rock solid, I think glass tape is enough.....

Then a good coating, and I don't think latex is a good coating for a tear, but that is just my opinion......

I agree. This is good advise...
Also I found resin at Wall Mart in the auto section. $8.00 a quart I used this on my end grain. Worked great. In my area True Value has an epoxy paint product You mix the hardener in the paint just before you apply. I am using this on my exterior side walls...

DEE

[Arne]

Interesting that True Value has epoxy paint... I'll have to take a look. That should do much better than house paint on a tear. One thing is an epoxy coating will not just cover like paint, but will help hold the pieces together, like an adhesive, and will also help seal any voids, like end-grains on ply.

Also, whatever coating we use also is on the roof, something a house does not have to deal with, plus the top of a tear will usually have a somewhat flat area, and a solid skin like an epoxy forms will protect better against standing water...

[brickz]

I did a combination. scarfed the joing, put a backer board down the inside length and then used 3" fiberglass cloth outside. TD's painted with two coats primer/sealer and 3 Alkyd coats. One down side with the fiberglassing and paint is you will always see the seam. tried to feather it out but that's still a lot of thickness added. I'm way more frugal than fussy so I'll trade off the strength over the "what's that ridge?" questions. I am going to try to hide it with the paint scheme a little. I just finished mine so I'll get back to you in a few years to let you know how it worked.
brickz

[doug hodder]

I joined my sheets of plywood using a 3 wing cutter on the router, It cuts a groove in each side that you want to bond. then I just cut a piece of maple for the spline and epoxied it all up. It's way tough. One of the advantages of doing it this way is that the 2 pieces will align better than by trying to scarf a joint. On a trailer side I would think that it would be fine, that's why I did it. Scarfing is the preferred method on siding a boat. For me, I just don't know if scarfing would be the way to go on something that doesn't have the stress of water pressure on it. but then I'm not a good person to listen to on that either as I've built a couple of boats in mohogany and didn't scarf them either. I just matched the grain of the wood and butt jointed the material, and backed it up with a backing plate......Just my opinion.........either way, I'm sure it will work out for you. Doug Hodder

[Arne]

I think I'd go with ship lap myself, being a novice wood worker (or being lazy might be more like it)..... I used scarfing on a kayak, but that was to make a long bendable piece with no backing at the joint......

All the recommendations are good, though, and all would work. Some might just take longer than others..... all would be strong enough on a stud.

I do think I'd stay away from a straight butt joint, though.