Need advice on how to repair a stress crack in my hot tub shell

[DangerBoy]

I've just discovered a small hairline crack in the (Endurol?) shell of my 1995 Hot Spring Grandee. It's only about an inch long and located on the vertical wall just below the top of the cooling bench where you step onto getting into and out of the tub (see photo). The crack maybe small right now but it goes all the way through and leaks. My index finger is pointing to the crack.

Watkins recommends cutting through the false wall in the pump compartment and patching it from behind/on the inside with Devcon Zip Patch. I was just thinking about sanding around the outside of the crack, drilling holes at either end of the crack to stop further propagation and filling the crack with some sort of strong epoxy like Marine Tex or another possible product is Plast-aid but I'm wondering what the experts on this site think of those ideas. I see the logic and merit in Watkins' solution in that applying a fiberous patch from the inside will help reinforce the area making the patch material stronger in tension than an epoxy filler but the downside is that I'd have to cut a pretty large access hole in the engine compartment shell to get at that spot and give myself enough room to work with my hands in order to adequately repair the crack from behind. The spot where the crack is would also be very cumbersome and difficult to get at from inside the pump compartment. My solution, if it will work, or if I can make it work, would be way easier and faster.

What are the chances of my idea of stopping further propagation of the crack by drilling holes at either end of the crack actually working? If I was to fill the crack with an epoxy of some type, what product would you recommend? I immediately thought of Marine Tex because it's super strong and hard and white and I've used it many times to repair chips and gouges in the gel coat of the underside of my boat's hull and it seems pretty bulletproof in that application. But the boat is made of fibreglass, and I believe the hot tub shell is Enduron which, if I'm not mistaken, is an ABS backed Acrylic material (have I got that right?). As such, the Marine Tex won't be subject to the same conditions or applied to the same material in this application. Perhaps the Plast-aid would be a better choice.

Whatever material I choose to make the repair with, my big concern is making sure it will stand up to whatever tensile stresses this location is subjected to, maintain the bond and seal and prevent the crack from propagating any further.

Is anybody able to offer some advice and/or relay what their experience has been in repairing a crack like this?

 

[DangerBoy]

I've just had a very interesting conversation with the gentleman who invented Plast-aid and pending verification of the compostion of the material my hot tub shell is made from, it sounds like it's just the right product for the job.

I believe/theorize the crack has appeared in this location because the cooling bench flexes a little when you step on it and I think that causes the area where the crack is to go into tension when someone puts a load on the cooling bench when stepping or sitting on it. Because of this, I'm concerned about the repair failing and the crack continuing to propagate under the stresses placed upon it through normal use of the hot tub. In an effort to avoid any of these things happening, I'm going to take extra measures to give the repair as much tensile strength as I possibly can. This will include fortifying whatever material I use for repairing the crack to give it additional tensile strength. I can document and report on the process and results if anyone is interested in following it. Let me know.

 

[santacruzpool]

Plast-aid is amazing stuff - I think it would be a great repair for the inside surface - it can be sanded and even polished a bit once it cures completely so that it blends in nicely. That would solve the leak and the exterior surface crack. In addition, I would look into adding a small fiberglass patch on the inside, behind the crack, to strengthen that spot - hopefully you can get to that area. Fiberglass repair kits are pretty common.

 

[DangerBoy]

The spot is very difficult and awkward to get at from behind. Given its location and how much stuff there is in the way of getting in there with whatever tool I use to cut through the false wall surrounding the pump compartment and get to the crack site, I'd probably have to remove just about everything (jet pumps, circ pump and heater, etc.) so I could lay on my back in there and be able to reach up into the work area. I could maybe do that but I'm thinking of going another route that will physically be much easier to accomplish.

Here's the problem I see: The crack has formed in this particular location because the area is subjected to tensile stress perpendicular to the direction of propagation. I don't think it was just because someone too heavy stepped on the cooling bench and it cracked all of a sudden; I think it happened because fatigue occurred in the material from repeated loading and unloading of tensile stress everytime someone got into or out of the tub over the last 24 years. It's like how you can get a strip of metal to break when you bend it back and forth over and over again. That's a fatigue failure. More or less the same thing happened here I think.

As Santacruzpool was alluding to in his comment, the big problem with doing this repair all on the outside is that it will be difficult to give the repair the strength it needs to withstand the same stresses that caused the area crack in the first place. I'm lucky that I caught it at just an inch long because if I hadn't it might've gotten a lot longer pretty quickly. At the same time, the forces that caused the crack to form will still exist after the crack is repaired so I need to make sure that whatever material I use to fill and seal the crack and the bond between that material and the acrylic substrate are as strong or stronger than the original uncracked substrate otherwise the bond or the fill material itself is likely to fail again.

If I could fibreglass the crack from behind, that would add a lot of strength and probably keep the area from failing again in the same way. But if that's not practical, how can I do that when making the repair solely from the front? I suppose I could try doing it with a big-arse fibreglass patch over the area but that'd look pretty crude and ugly. I'm trying to find a more elegant solution that will be fairly hard to see yet be as strong or stronger than it needs to be to keep from failing again.

So again, two things need to be very strong here: the bond to the substrate and the fill/repair/bridge material itself. Those are the two places the repair can fail.

Since Plast-aid is actually acrylic and is said to form both chemical and mechanical bonds with the underlying acrylic and ABS subtrates, that should provide a better bond than any epoxy which will rely only on mechanical bonds with the subsrate. The question then becomes, how can I possibly increase the tensile strength of the Plast-aid to make it as strong or stronger than the surrounding substrate? I believe the answer to this is to add fibres into the mix.

Most people know that the high tensile strength of fibreglass or carbon fibre composites comes almost exclusively from the fibres, not the resin matrix. The resins used in those composites are actually fairly weak. If Plast-aid is acrylic, it should have close to the strength of the original substrate but not necessarily quite as much strength. It depends on how the polymers are linked and cross-linked etc. So for the sake of arguement, let's say Plast-aid is almost as strong as the original substrate but not quite. I reackon (and the guy who invented Plast-aid agrees) I can increase its strength by adding some fibres into the matrix of the Plast-aid - essentially making an acrylic glass or acrylic carbon fibre composite. Getting ahold of glass fibre is easy and that's what I had originally planned to use but a local industrial plastics shop has kindly offered to give me a couple small scraps of carbon fibre matting to try so that's what I plan to use.

So that's the plan basically, drill a couple small holes at the ends of the crack to hopefully stop the crack from advancing, grind the crack out a bit with a dremel and sand a bit to gain some more surface area for the Plast-aid to fuse with/bond to and infuse the Plast-aid matrix with some carbon fibre to (hopefully) give it a bunch more tensire strength and sand it down to make the repair blend in as much as possible. Then just cross my fingers and wait to see if it holds up long-term.

It should be an interesting experiment. I may even make a You-Tube video out of it so people can learn from what I did (or avoid doing what I did if it doesn't work!).

 

[santacruzpool]

Looking forward to seeing your final repair - it sounds like you have a good plan - good luck!