Construction adhesives in deck replacement

Hello everyone. I'm currently restoring a small boat and was wondering if there is any benefit or problem with using some type of construction adhesive (PL, Liquid Nails etc) to help secure the deck to stringers and edges. It will all be epoxied over with a layer of 1708.

Will the chemicals in the adhesive react in a negative way with the new stringers or anything of the sort that I'm not thinking about?

Does anyone have any preference for an adhesive for this sort of task?

 

Epoxy

 

Some small boat builders have used construction grade adhesives (PL, etc.), but these aren't the best choices, as they're designed for static loads, not the dynamic ones we see on boats. They're really good hold faux wooden panels to your aunt Millie's family room walls though.

What are you securing? If it's plywood, it should be epoxied before it's installed. The adhesive on the stringer tops can live with PL Premium, but the edges and seams are where the issues can come back and haunt you. If it's getting skinned with 1708 (why?), I'd just use epoxy to bond it in and seal seams and edges.

 

Maybe you guys misunderstood. The marine grade plywood I'm using for the deck will be completely encapsulated in epoxy. I even go farther than most and completely glass the bottom of the deck as well with a layer of 6oz cloth. I'm am not just using construction adhesive to stick a piece of raw wood in my boat.

Generally most boats I've worked on just have the deck sitting on top of the stringers with nothing bonding them at all and others unfortunately use screws which make later removal of the deck a pain if you aren't expecting them. I don't ever visualize anyone needing to remove this deck but if they do it would be nice if they didn't have to destroy the stringers to do so, therefore in NO WAY am I going to epoxy the deck to the stringer itself.

I was just considering using a bead of glue or maybe even silicone as a cushion that would help keep things in place while I'm glassing and tighten any gaps or voids between the two fully epoxied sections. I'm NOT counting on it to be what hold the boat together!!!!!! Thanks

 

I applaud you for glassing the underside because water most often finds it way in over time and starts rot from the bottom side. All the top side glassing in the world doesn't keep it from happening.

Boat mfgs were gluing liners down on stringers in the late 1970s and still continue. The stringers on a common 25'-30' center console are boxes (12" wide typ and of 1/2" ply) that provide a large contact surface for glue. They spread thickened epoxy on the stringers and set the liner down. It cures and there is no way to get the two apart without major destruction. I spoke to Proline in the mid 1980s for a cuddy cabin redo on a 21 walkaround. They told me 40 gals of epoxy were used to glue the deck/liner to the hull.

With that said, I use pt ply, glass on the bottom and set on dimensional stringers with thickended epoxy. I also drill and pull down with #14 wood screws. I fill the screw holes with epoxy first and recess the heads deeply to be filled later with epoxy. With pt I don't worry about water rotting anything but with regular marine ply I would do the oversize hole, fill with epoxy and drill for the screw to ensure water doesn't get to wood around it. Also, car batteries make excellent weights for setting floor when doing this. I'd say to at least use some type of adhesive when setting the deck down. PL was very popular 20 yrs ago and many home boat builders were using it with great success on small skiffs, etc. I haven't kept up on PL so can't comment on longevity.

 

Very few (read just a handful) of production builders "epoxy" stuff down to stringers. Some do use a Plexus style of adhesive to bond liners, soles, etc., but generally bonding as previously described, was done with polyester and vinylester based adhesive mixtures, not epoxy. Only recently have epoxies made serious inroads into the production builders market.

I find concrete blocks are much better than car batteries, as weights. They weigh about 35 - 37 pounds each, are uniform in size, easy to stack, don't leak anything, can be rolled onto any side, etc. I literally have a stack of them for just this purpose, outside the shop.

PL works in small craft, but not especially in large craft and doesn't tolerate dynamic loading very well. I have pictures of bulkheads, that have been skinned with PL as the adhesive, the 1/2" skins sheared their glue lines in a year, on one 34' power cruiser I repaired. Clean glue line failures in all examples.

If you want to "bed" the deck, consider a polyurethane or a polysulfide as mechanically fasten the deck down. 3M-5200 is pretty aggressive and likely stringer damage will occur if the deck eventually get pulled, so maybe a less aggressive adhesive, like 3M-4000 or 3M-101 (my choice) I too would suggest you're doing a good job, as moisture does seem to work up, from the stringer faying surface, into substrate above and naturally the stringer below. The light cloth will help greatly in preventing this.

 

I don't see epoxy being used to bond stringers and similar items in production boats, it's either old school glassing, Methacrylate adhesive, or Polyester/VE bonding putty, and possibly a combination of these. I'm sure some builder at some time did use epoxy, but it's not common.


Back to the question.

Anything strong enough to hold up long term will probably result in some damage and hassle to undo in the future. Look for a low strength high elongation product, this will at least do an OK job of keeping water out of the screw holes.

 

Thank you all. I think PAR hit the nail on the head with 4200 or something of that sort. It's a 16' boat with an open bilge between the two stringers and this is just to seal and fill any voids between the deck and stringers to keep moisture out of the rest of the substructure. I may countersink a few small screws to pull things down tight and prevent the deck from slapping against the stringers over time but hate to drill into the sealed stringers.

 

I'd use 3M-101, for it's less aggressive adhesive properties. It still sticks good, but is more sealant than adhesive. 3M-5200 - 4000 and similar are more adhesive than sealant. Check the 3M marine sealant product line and compare the "hardness" figures. 3M-5200 dries a lot harder than most realize at about 70 (Shore A), which is pretty rigid as sealants go, while 3M-101 is about 50, which is what most caulks are.

 

If you glass the bottom of the deck any moisture intrusion will have no place to go, which will rot the deck.

 

There should be no water intrusion in a well built deck. Also, if there is water the wets all the layers of the plywood, it won't dry through the veneers. That is, moisture won't travel through the glue lines in the plywood.