One of the things I wanted to try was attaching a strip of Sanalite (that plastic you use to make cutting boards) to the bottom of my keel to act as a sacrificial strip to protect my wooden keel.
So now I have 16 feet of 2 inch wide and 1/2 inch thick plastic and I am trying to figure out how to attach it. It is in four 4 foot pieces.
The guy at the plastic store said that no glue will really work on it so I was best to attach it mechanically.
I am planning on pre drilling the plastic with counter sunk holes for wood screws and drilling out the keel with oversized holes that I will fill with thickened epoxy and then drive the screws into the hardened, thickened epoxy. Make sense? I don't want water to get at the wood.
Here are my two questions (other than is this even a worthwhile idea):
1. How important it keeping everything really smooth on the keel? Will it make a big difference if I fill all of the countersunk screw holes to keep them flush, or is this just getting too picky?
2. Should I put down a layer of 3M 5200 (or something else) between the plastic and the keel to fill in the gaps (if there are any)?
Thanks in advance,
Steven Goodman
(future captain of the s/v Isla)
The plastics store guy is correct, nothing really sticks to HDPE or UHDPE (cutting board stuff). Silicone will stick a little and is what is used to seal edges where is may be attached to something, but it doesn't really do a very good job.
Since it's an inert material (you can eat it and it will not hurt you, though it's not particularly tasty), moisture, abrasion or other abuse will not hurt it, so a bedding really in only necessary if you expect your fasteners to fail and let moisture into the deadwood.
You can't drive screws into hardened thickened epoxy, it will just crack, split and chip out. You have to insert the fastener while the goo is in the green stage (still soft). I prefer to wait until the green stage when possible extraction is planned, but many insert just after filling using a release agent (wax) on the screw's threads.
Drill a hole twice the size of the screw shank (the shank, not the threads). Wet out the hole well with unthickened epoxy and let this get tacky (the epoxy feels like a gel, not liquid) then wet it out again with unthickened epoxy, before filling the hole with a thickened mixture to the constancy of peanut butter. Wax up the screw threads if you want (I don't bother and they hold better without it) and place the plastic strips on the deadwood. Weight it down or use tape to keep it from squirming around then insert the screws. Kind of screw them in by hand so they carve some threads as they go in. Let it sit for a few hours then use a screw driver and snug them a little, not too much, the glue is doing the holding (maybe a 1/4 of a turn), the fastener is just a "bracket" of sorts. Wipe off excess goo as it oozes out around the fastener head.
I wouldn't bother with a filler over the screw heads, as it will not stick to the plastic either. Just set your counter sink depth so the heads are at little past flush. If you have an irregular surface under the plastic, enough to cause it to wave and not lie reasonably flush then use some 5200. Use a generous bead(s) just before you tape or weight down the strips for the screw insertion part of the process. Wipe up any ooze out with an acetone damp towel. To get the 5200 to cure fast, drape a damp towel over the work, being careful not to let it touch the wet 5200. The moisture in the towel will "kickoff" the 5200.
I don't know how well thickened epoxy holds screws. The way I was taught for something like this was to drill holes large enough to drive in wooden dowels, which would be coated with epoxy. Then screw into the dowels. If any rot develops around the screws, it will be confined to an easily replaced dowel.
Epoxy can increase the "pull out" potential (strength) very substantially (read many times over other conventional means). The wet epoxy will leach into the wood fibers, forming a mechanical linkage with the wood, which then has a screw "bonded" to it. A ten fold increase in pull out, can be seen if done properly. This is called fastener bonding and can be done two ways, one is if the fastener is expected to be removed periodically and the other if the fastener isn't. In both cases, the fastener can be removed, but if it's planed for, it's easier, but not by much if proper technique is employed. Log onto West System's site and check out their test data on some big bolts used to hold huge loads, with and without bonding techniques used. The bonded bolts actually broke the test equipment. The added benefit is a heavily coated, epoxy hole that will prevent moisture from getting into your deadwood.
Thanks Paul. Just the kind of info I was looking for.
Any suggestions as to what to use to put the unthickened coats of epoxy in the holes? These are pretty small holes. Do I use a Q-Tip?
Also, I was going to try using a syringe to put the thickened epoxy in the hole so I am sure to get it right down to the bottom. Is this the best method?
Thanks again,
Steven Goodman
I use a squeeze bottle and sometimes a syringe to squirt goo into holes. I like the bottle better (especially if it has beer in it), but it doesn't work well on small holes and can make a pretty good mess. Another trick it use gravity. Dip a tooth pick in some goo and hover it over the hole. The drip will run down the tooth pick and drop off just where you want it. You can poke the goo into the hole once it's dropped off. Wetting out holes can be done with a Q-Tip is guess, but I generally use the "force it in" method, like jamming a squeeze bottle against the hole opening and squirting more then necessary. I've also used pipe cleaners too. Whatever works for you is the best method.