I’m looking for some advice as to what is a good adhesive for attaching D-rings (Harmony D-Rings on plastic patches) to the INSIDE of my kayak. I have a couple older rec boats I loan to people new to kayaking. I just moved from the midwest where we paddle streams and it was easier to retrive a sunken boat in still water, to N.C where I paddle a lot of lakes. Two of the boats do not have any seal compartments and I want to put float bags in the bow / stern and need something to attach to, so they would be easier to bail out or recovery if flooded. I have bought some Harmony D-rings on the plastic patches but need an idea as to what would be the best glue to attach these rubber / plastic patches to the plastic rotomold hull on the inside.
G Flex epoxy would probably be best
I have had some success bonding vinyl patch-mounted D-rings to polyethylene with vinyl adhesive, but the bond is not as secure as it is to Royalex. Contact adhesive is another alternative for a light duty D-ring but it will separate if subjected to any strong distracting force.
If you want to assure a strong bond I would use West Systems G Flex epoxy. You can buy a small amount from NRS or Jamestown Distributors for about $25 or so. You can mix it up using equal volumes of resin and hardener by eye.
To get the best bond to polyethylene mark carefully the position of your patch with a marker and clean the surface well with alcohol. Either isopropyl or denatured EtOH will work. Allow it to evaporate fully then carefully flame the surface to be bonded using a hand-held propane torch passing just the tip of the inner blue cone of the flame over the surface. You don't want to heat the surface much at all but the flame will temporarily oxidize the polyolefin making it more receptive to bonding to the epoxy.
Apply thin coats of epoxy to both the hull and the patch. It will take up to an hour or so for the epoxy to cure so there will be no immediate tack bond. You need to hold the patch in position with something like strips of duct tape till the epoxy cures. It is also good to apply some even weight to the surface of the patch using something like a small bag filled with water or sand.
Mechanical attachment
pblanc’s answer is perfect if you are going to go the glued-in D-ring route. There are a few other adhesives that would also work, but are harder to get then G-Flex.
If the boats already have some sort of deck rigging it would be better to attach P-clips using the existing hardware (you might need longer screws).
For the rear corners of the bag, I used
the front of the foot rest rail as an attachment point. The front end of the bag probably can’t go anywhere as long as it’s inflated, but I had a Harmony drain plug kit and installed that way up in the bow, using an attachement ring glued into the screw plug as an attachment point. The drain plug can be handy also as a way to drain residual water out of the nose of the boat, which is a Necky touring kayak.
You can use pop rivets plus sealant for attachment, though you should keep the attachment point abobe the usual water line.