Replacing bilge switch, and filling old mounting holes.

[Sailing Amok]

Hey guys! Well, I decided to try filling the bilge with clean water, to make sure the old float switch was working, and... it ain't. I've ordered a new switch of a superior design, which means unfortunately it won't line up with the current mounting holes from the old switch. I've read a few previous posts and seem to have a few options. As I've never done any epoxy work, or even bedding of hardware, it's hard for me to determine the best approach.
It looks like the current switch is screwed directly into the floor of the bilge, so when I remove it I will need to fill those holes. I'm guessing epoxy would be the best way to go, or is this overkill, and I should just squirt some 4200 in there?
Once that is dealt with, it comes to mounting the new switch and opinions seem mixed. Some folks suggest drilling oversized holes, filling with epoxy, drilling pilot holes into the epoxy, and then screwing the switch in. I've also seen 4200 suggested for this purpose, which I don't really understand. Do you just fill the hole with the 4200 and push the screw in while it is still wet? Or do you let it dry and then screw into it? I can't imagine how one would screw into something like that, or that the screw would gain much purchase.
Others suggest simply sticking the switch down with 4200 and drilling no holes at all. Having never worked with the stuff, how hard will it be to remove the switch if stuck on with 4200 when the new switch inevitably dies?
A third option seems to be glueing down a piece of plastic with 4200, and then screwing into that. The only disadvantage I see with that method would be raising the switch slightly, and thereby increasing any standing water in the bilge. If I were to go that route, what sort of "plastic" are people describing? I guess it would need to be at least 1/4 inch thick for the screws?
Thanks!

[scgunner]

Aaron,

You may be making more out of this than you need to. All the options you mentioned will work because mounting the bilge which isn't super critical, you could use Velcro stick-ons and it would work, although I wouldn't recommend that.

If it was me I'd fill the holes with West Systems epoxy using a 407 filler, then just drill holes for the new switch. I'd use short screws so you don't go any further into the gel coat than necessary.

[waughoo]

I would agree with the above but add a few more details to clarify.  Drill pilot holes for the screws you intend to use for the new switch and then put a small dab of 4200 on the pilot hole, then mount the switch using the screws.  The screws do most of the work, and the 4200 is just a bit of extra insurance and seals the hole to avoid water migration.

[Sailing Amok]

I would agree with the above but add a few more details to clarify.  Drill pilot holes for the screws you intend to use for the new switch and then put a small dab of 4200 on the pilot hole, then mount the switch using the screws.  The screws do most of the work, and the 4200 is just a bit of extra insurance and seals the hole to avoid water migration.

Ahhh I think I understand now. The 4200 just gets carried down with the screw, sealing the threads as it cures. It's not forming the material being screwed into like the epoxy method would be.

[waughoo]

That is correct. 

[KWKloeber]

Aaron & Kristina

A couple thoughts - as with nearly everything "boat," there is not one single correct answer. 

I agree with you -- no plastic base unless the float itself can dip lower than the plate the switch is mounted to.

Naturally, the old holes need to be dry before filling them. ​

All of this is making way too much of filling and waterproofing a screw hole but doing it will introduce you to using epoxy -- which some knowledge of is always a good thing to have in your wheelhouse.

I would fill the holes with epoxy resin (and hardener) - additive (thickener) isn't required in this application and I would say is (very slightly) better w/o it.  The reason is resin will self-level easier than with thickener in it, and it will seep into every recess in the screw hole (poke the partially filled hole with a toothpick to escape any air trapped below the epoxy then finish filling.)  Use a West systems syringe, or just drop epoxy in with a toothpick or whatever.  NOW, again realize that you are just filling a screw hole so it's no big deal, just something to learn.

Likewise IIWMB I would "pot" the new holes with epoxy as you correctly described and redrill to install the new screws.  That's overkill, but just the way I do it.  It would be a great opportunity to learn epoxying with that technique in a location where it isn't cosmetically important and you really can't screw it up.
​
Using 4200 or any waterproof caulk is fine -- but you want to prepare the hole first. 
Put a countersink in the floor after you drill the pilot hole, to form a reservoir for the caulk when mount the switch (or bed any type of fitting.)  Fill the hole and around it w/ caulk and when the base compresses against the bilge, the caulk gets pushed into the vee (the countersink) and compresses against the screw and switch and bilge floor.  Tight fit; no water gets past.

Another option is to prepare as above and use butyl tape instead of caulk.  That would be my second best choice over epoxy-potting the holes.  You roll out butyl into a small sausage and put a ring tight around the screw head and under the base tight around the screw.  Tighten down and the butyl gets compressed into the vee and seals the hole.

-ken

[Ron Hill]

Aaron & Kristina : Look at just using one screw to hold the new float switch in place.  Use one of the old holes and fill the other. 
All you need is to hold the switch body in place so the float can go Up and Down!! 

Just make sure that that float switch is somewhat in the middle of that bilge compartment.

A thought

[Sailing Amok]

Thanks again guys. I think Ken is right about this being a great opportunity to learn epoxy, in a low consequence situation. From what I understand, the slow hardener is recommended for filling things like screw holes, as the fast hardener can result in runaway exothermic reactions and bubble production. Up here in Thunder Bay, days with the minimum recommended working temp of 16c for the slow hardener are hard to come by. If it's above 16c, I'd rather be sailing. I'd guess the temp in the bilge is probably something like 10c right now. Would the exothermic reaction of the epoxy effectively compensate for that though?

[waughoo]

I used the fast hardener when it was pretty cold out.  What youre doing is so low impact that it is less likely to be any real problem if you get a thermal reaction.

[scgunner]

For that matter while you're filling the old holes you could just put a few dabs of epoxy on the switch base and glue it down, then you won't have to worry about new holes.

[KWKloeber]

For that matter while you're filling the old holes you could just put a few dabs of epoxy on the switch base and glue it down, then you won't have to worry about new holes.

That highly depends on which plastic -- there are ones that epoxy will not bond to, or to attain a minimally acceptable bond requires pretreatment (like surface flaming) to modify the molecular structure of the plastic.  It depends on the chain structure of the particular plastic.

[KWKloeber]

From what I understand, the slow hardener is recommended for filling things like screw holes, as the fast hardener can result in runaway exothermic reactions and bubble production. 

I'd guess the temp in the bilge is probably something like 10c right now. Would the exothermic reaction of the epoxy effectively compensate for that though?

A&K,

I have had no issue like that with screw holes.  The run-away is when I've had a LARGE volume in a DEEP (relatively to surface area) container.  But we're talking a tiny volume w/ an admittedly colder adjacent area (a heat sink, as it were.)   I use plastic bathroom cups a lot for mixing - forget trying to use a full cup w/ fast hardener - you'll have a solid block in 2 minutes.  In fact yesterday I used fast hardener on a high 70Fs day -- but the application was (relatively) low volume (thin) to surface thin and I wanted a quick outcome.  Getting friendly w/ epoxy has other benefits than just around boats - yesterday I salvaged "historic lumber" by scarfing in a repair (using microfiber filler) to replace 3-foot of a rotted edge.  Although I could have done something else I wanted to keep the original lumber (71-yr-old "real" 1x10 a full 7/8" thick!!)  So I used a dollar of epoxy instead of $50 USD of pandemic lumber.

I have used it in colder than recommended temps w/ no issue (repotted EVERY deck thru hole and rebed everything on a J/120 one weekend in temp you describe) -- it just took longer to cure (I got a blessing from the Mas Epoxy tech gurus first.)  You can warm the area with a heat gun, but the huge heat sink of the keel stub won't allow that to last too long.  BTW, West is certainly fine stuff, but look at Mas Epoxy - the mix ratio is more forgiving (2:1) and the hardener is non-blush.

Many times epoxying (non-critical applications) is kinda like a cheeseburger.  Not too particular - Not too precise.  Try not to overthink it too much.  Go for it.

[Ron Hill]

Aaron : I'm going to guess that you are trying to "Make a Mountain out of a Mole hill"!!  Unless I'm wrong, these are TINY holes no more than 3/16" diameter and probably no deeper than 3/8" and the bottom of the hole still is solid!

The most important thing for you is to make sure that the hole is DRY with no oil or grease in it.  So just mix up a very small batch of epoxy and fill the hole.  Take a small tooth pic and swirl it around in the hole so the epoxy is knowingly touching the side and make sure the hole is just filled to the top.

I'd still use one of the holes and its screw to secure the float switch and fill the other. OR You could take the screw w/caulk and fill that hole with the screw!!   

Your boat so do what you want!! 

A few thoughts

[Sailing Amok]

Aaron : I'm going to guess that you are trying to "Make a Mountain out of a Mole hill"!!  Unless I'm wrong, these are TINY holes no more than 3/16" diameter and probably no deeper than 3/8" and the bottom of the hole still is solid!

Ron, you're not wrong, I'm definitely making a mountain out of a mole hill. I'm really just using it as a learning opportunity in a low consequence scenario. No point in practicing a technique if your practice is mediocre. "Il meglio è nemico del ben" and all that, but I'm gonna make these the best potted screw holes I can.

[KWKloeber]

I'm gonna make these the best potted screw holes I can.

If you are going to pot the NEW screw holes, see here for an excellent how-to pot:
https://marinehowto.com/sealing-deck-penetrations-to-prevent-core-rot/

These are for through holes, but the tips are still helpful.

There's one thing you need to know about potting screw holes vs bolt holes.  Expoxy resin (unlike wood) has no give - you can snap off a stainless screw very easily when driving it home.   The pilot hole has to be very carefully sized so the screw thread cuts in but doesn't bind otherwise the head can shear off.   Screws into epoxy isn't great.  One way to fix that is to not install the screw into pure resin or with a hard, structural filler (like microfibers.)  If the hole is potted and then drilled and filled using a low density filler (west #407) then the thread can cut into the pilot hole w/o the head shearing off under torque.