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PL Premium Construction Adhesive:
This is a polyurethane based adhesive manufactured for the construction industry. I learned about it from Herb McLeod which he's used on several projects including his Michalak designed AF3.. It is extremely strong and water proof. It is in the same category as 3M 5200. The manufacturers do not recommend it for continuous contact with water. Here is their data sheet on it.

PLPremium 2002.pdf

Here is a technical review of Polyurethane glues in general. They measure up very well against Resorcinol, except in the boil test. I recommend avoiding boating in boiling water.


What's all the fuss about this stuff?

PLtubes.jpg (16287 bytes)

Many backyard boatbuilders have been using this stuff for some time. Its available at many of the chain home centers like Home Depot, Lowes & Scotty's. Mike Goodwin, who has used hundreds of tubes of the stuff on construction & boats,  has this to say about it:

"This picture shows the 2 types I use , I recommend the Concrete version for boats work .

The Concrete version says textured on the label , but comes out clean and smooth and does not bubble up .

The bubbly stuff is more brittle when cured , the other flexes like hard rubber.

I find the PL construction adhesive bubbles the more you work it , put it down
and leave it alone , dont try to fillet with it unless you can live with the
bubbles . I have laid down beads of it in a corner with no bubbles , work it with a tongue
depressor into a fillet and bubbles galore . I think the bubbles come when it
gets to a thin film .

PL Concrete formula does not foam or bubble , just like 5200 , and sometimes I wet
the joint areas with a plant sprayer to increase bond and speed cure .
Would I use it for scarphing ? depends on what I'm scarphing and how it will be
used in the boat . I have used it to make scarphs in structual timbers.

Miketest.jpg (17621 bytes) This picture , shows a joint I glued together , a 2x4 crossing a 2x6 , and tried to take apart . It took a Sawsall , a crowbar , a 5lb hammer and a chisel to get it apart and consumed 30 minutes . That was the weaker general construction adhesive too .
PLluantest2.jpg (7165 bytes) Side Bottom Joint Test -

This is an informal but informative experiment I did on some 1/4" luan scrap. (Not marine ply.)  A small simulation of a side joining a bottom of a boat.  The pieces were 9" long - the side was 2" wide and the bottom 3" wide. I glued them at a 110degree angle and ran a bead of PL down both sides of the "side" piece and made it into a fillet with a plastic spoon. As you see it dried bubbly and ugly. (Next I'll try the Concrete version.)

PLluantest.jpg (11091 bytes)
PLbreak1.jpg (9773 bytes) I put it on the floor "tent like" and stood on it. (I weigh 180 lbs) It flexed but didn't break. I then tried to close it like a book, with my hands and the ply delaminated with a loud snap. PLbreakclose.jpg (11426 bytes)
Some Glue Joint Theory...

filletsections.gif (3560 bytes)

Whether a flat joint or a right angled joint the purpose of all joint systems, butt block, chine log, the fillet and the glass as I understand it is to distribute the anticipated force over a larger area. If our 1/4" ply side to a bottom pieces were 4" long - (for easy math) and we only glue the edge, we have 1 square inch of glue area in that joint. 
    Say you add a fillet of PL (or epoxy even) with a 1/4" radius (radius minimum usually equals the thickness of material being joined) Now, with the 1/4" of glue running onto the bottom and up the side you've increased your glue area to 3 square inches. Add a fillet on the "other side" of the side and you now have 5" of bond area. 2 inches of glass tape on one side gives us 9" square of bond area. If we tape the outside too we get 17" square inches (I'm not adding badly, you only count the 1/4" at the bottom of the side once) A chine log of 3/4" x 3/4" would give us 7 inches of glue surface. You get the picture.
    In my test break the wood failed before the glue, but if we encounter enough force to fracture that amount/area of wood we'll be wet just the same.
    So it is a balance between what forces we anticipate our boats will
usually encounter, and what forces they might occasionally (or rarely or
never) encounter, and how much you want to prepare her for.
    Since we're not engineers, we either have to trust tradition and do what
we have seen succeed before, or do our own testing and hope our testing
methods are reliable.
I find myself asking.... do we care whether glue or wood fails if we are suddenly treading water?
Another approach...

A friend of mine has successfully (so far) made use of this joint configuration. An inner bead/fillet of PL and outer epoxy and glass tape. The little test I just ran shows the wisdom of this. The glass could prevent the plywood from delaminating at the edge.

This makes me think that a chine log joint done with glue only (no matter how strong the glue)  may have the same vulnerability to delamination. The advantage of ring nails or screws is they continue to help keep the ply edge from delaminating.

What about using PL and glass?

A few folks, like David Colpitts of Mouse Boat fame, are exploring this area. I've done one experiment with it on the inside bow "stem" of an 11.5 ft double paddle canoe I made. You can view that test here.

David made a kayak out of PL and the glass mesh tape sold for drywall construction. That was 5 or 6 years ago and it's still alive and kicking.... er floating... See his testimonial here.

What about using PL for scarfing plywood? A report from Mike Saunders.

I did some scarf and glue testing. I made some scarfs on 1/4" luann
at about a 10 degree angle. That works out to about6-1 or something
along those lines.

I did one with epoxy and one with PL Premium. after 24 hours I took
both scarfs and placed them on the handrail of the front porch right
at the scarf. After scarfing the pieces were about 12" x 6". I
applied pressure. I broke both at the surrounding wood. Neither glue
joint failed. The next step is to do  some 3/8 SYP ply and do the
same thing.

I also made a test filet with the PL and like most people have had
luck with, it bubbled. But the joint was strong. I was impressed.

I scarfed some 1x2 material with my sliding mitersaw. That was a
cinch. I placed a 2x6 alongside the blade and clamped it down. I
moved the angle to 7 degrees (8-1 ratio), and placed the 1x2 into
the cutting area at the right angle that the 2x6 was holding for me.
This resulted in a simple and beautiful scarf on the 1x2 material.

I'm going to work at making a fixture to hold my circular saw to cut
the  scarfs in ply panels. I will post pix of all this when I get
done with the testing. [and we'll add them to Mike's Tips page. ed.]

I was really impressed with the PL premium. I will be using a lot
more of it. 


As yet un- tested....

This idea is a bit half baked but I'm trying for a joint that will be quick and easy for making "toy" boats like one sheet skiffs, canoes and the like. In this sketch the ply ledge has been covered with PL too and then the whole unsightly mess has been covered with some sort of split tubing. The tube could also double as a kind of chine rub rail protecting the edge of the ply from collision or grounding damage.

PLtest2.jpg (8077 bytes)

Another unscientific but interesting test of PL Premium's strength.

I used two 8" pieces of 2x6 pressure treated lumber. I buttered the end grain of one and hand pressed it into the middle of the other forming an inverted T. No clamping, and no fasteners. Let it set overnight. I now challenge anyone to break the bond. Anything goes (except driving over it with your car) you can use a wall, stand on it, throw it on the concrete or whatever. If anyone succeeds in breaking this bond, please let me know how you did it. (Did I mention I love this glue!)


Ccm00010.jpg (11802 bytes) It's good to keep cleaning supplies around when working with PL Premium. It's easier to work with than epoxy, but I'm told it's still quite toxic. Some people have allergic reactions to it, like rashes and hive like symptoms. Please be careful. Rags and paint thinner will come in handy for cleaning up your tools. WD-40 is also very effective for quick clean ups.  Wear gloves and keep it off your skin. If you inadvertently get some on your skin, immediate use of WD-40 followed by soap and water works great.

PLmasonryfillets1.jpg (10634 bytes) So this is PL "Concrete Crack and Masonry Sealant" - It has the word "crack" thrown into the label, but I'm hoping its the same stuff. I have caulked both sides and filleted them with a spoon and through a film of plastic wrap.
PLmasonrytube1.jpg (15896 bytes) Here's a picture of the tube.
PLmasonryfillet2.jpg (11000 bytes) And, believe it or not, this is the "after"  photo. And Mike was right. No bubbles, looks the same as it did last night. I tried to peel the plastic wrap off the next morning and the PL was still sticking to it. It smoothes so well without it that I don't think it's necessary.

The tube claims a 24 hour cure time and a week before painting, though no priming is necessary.

PLMasonryflex1.jpg (7163 bytes) This is the before and after shot for the flex test. You'll have to take my word for it. It returns to the same angle after flexing it through as much as 90 degrees.
PLmasonryflex2.jpg (7421 bytes) Here it is flexed. (See above for the "after" picture.)
PLmasonryflex3.jpg (6059 bytes)  After many "hinge" cycles (sorry I didn't count them) the joint failed. The wood didn't fail, the PL released. 
PLmasonryfail2.jpg (13392 bytes) Here's a close up. It sort of peeled off the surface of the wood with just a few small fibers still attached. If this was used as 3M 5200 or Sikaflex is used in lumberyard work boats - screw and nails are used as well. And in combination with those mechanical fasteners will probably never be a problem. For those of us who are exploring adhesives only assembly techniques.... this gives us pause.....
Musings and

Tentative Conclusions?

The masonry stuff is sold as a sealant and not an adhesive. While their construction adhesive makes claims like "will last as long as the surfaces it joins together."

The slight difference in the name of this Masonry formula may mean a difference in the formulation from the type Mike Goodwin has such great luck with.

I'm going to try a combination of the two. First a bead of the adhesive, and either leave it alone or smooth it only slightly through plastic. Once cured follow that with the Masonry stuff for a smooth paintable fillet. The two steps and double cure cycles might end up making it more trouble than it's worth. Unless you just hate working with epoxy and glass as much as I do....;-)

I'll keep you posted.

Mike Goodwin's feedback...  

"I would have let it cure longer (48hrs) especially
with the plastic wrap which may slow cure . The stuff I have put together with
it would not flex that far .
Did you push it down in a bed of the stuff or just bead both sides ?
I have found it next to impossible to get out of the woodgrain once cured , so
I think it may have not cured completely .

Mike G"

I have some more tests in the works, this time with a bit more patience.

Bad Luck with PL Premium....

I tried the Colpitts fiberglass scrim drywall tape and PL Premium in a
test with two 1 ft square scraps of 5 mm luan.  placed the luan ply at
right angles, used two layers of the tape on the interior seam, (no
filet underneath. pressed in the Glue, then smoothed with a putty knife.

Let it dry for 24 hours, then with my hands broke then tore them apart,
on the seam, leaving tape and PL glue attached to both sides

It may not be a fair test, but I decided that it was too weak  to rely on.
Scott Jordan

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