anode studs in GRP hull

[Carols]

Any comments on what is the best to use for anode fixing studs in hull silicone or sikaflex

 

[jfm]

Any comments on what is the best to use for anode fixing studs in hull silicone or sikaflex

Polyurethane, not silicone. I can't really think of any 24/7 underwater application where silicone is the right answer. Sikaflex make both so the answer to your q can not be "Sikaflex". I would use 3M's 5200 product, which is a bit like waterproof welding

 

[Tender_aft]

waterproof welding

Hahaha Did you invent that phrase or do they call it that?..... that's a great tag line for it!

 

[superheat6k]

Sikaflex 291.

Apply plenty around the hole inside and out with large penny washers either side. Tighten gently so a good squish appears all around the washers on both sides, but don't squeeze too hard to force all the sealant out. You want about 2-3mm layer beneath the washers.

Allow the sealant to cure for a week or so, then tighten quite hard.

When you apply the zincs do not allow the posts to turn or you break the seal.

 

[jfm]

Sikaflex 291.

I don't agree. Both 3m 5200 and Sika 291 are polyurethane. Sika 291 is however a general purpose medium strength product, rated 260psi tensile strength. Its equivalent in the 3M range is 3200. It is ideal for lots of general jobs, like say underwater lights, bedding deck fittings, etc, where you need a good seal and a medium strnegth joint that you can break again if you need to

In contrast 5200 is an uber strong polyurethane, like welding as I said. It is rated about 600psi tensile. If you used it to stick underwater lghts, which have a big flat area, you'll struggle ever to get them out.

3M 5200 is so strong that you shouldn't use it if you don't want its strength. 3200 and 291 are much better for most jobs. But for this application, studs for anodes, you do indeed want the "welding effect" else you risk the studs turning when you undo the nuts. I therefore strongly recommned 3M 5200 here, not sika 291

 

[VicS]

for this application, studs for anodes, you do indeed want the "welding effect" else you risk the studs turning when you undo the nuts. I therefore strongly recommned 3M 5200 here, not sika 291

Sikaflex 292 then!

 

[superheat6k]

In my view the important thing is the I initial seal and not to disturb it. This means someone holding the inner stud still if necessary.

However, I replaced the anode studs on my boat last year and sealed them exactly as described, with 291. I then secured the anodes outside the inner securing nuts with doubled nuts -I plain, 1 nylon. Nipped up moderately hard. This year the nuts freed easily, but none had slipped at all, and when I did the nuts back up with the new anodes they didn't even flex, let alone move.

 

[jfm]

In my view the important thing is the I initial seal and not to disturb it. This means someone holding the inner stud still if necessary.

However, I replaced the anode studs on my boat last year and sealed them exactly as described, with 291. I then secured the anodes outside the inner securing nuts with doubled nuts -I plain, 1 nylon. Nipped up moderately hard. This year the nuts freed easily, but none had slipped at all, and when I did the nuts back up with the new anodes they didn't even flex, let alone move.

This is getting daft. You are now arguing using one isolated example to prove a general principle. If your threads happen to run easy because you've luckily not had them corrode a bit or the boat has only been in the water for a year, or you used uber quality greaseor whatever, you could probably use blu-tak. However, that isn't the general case. In general, threads seize, maybe cos the boatyard used nasty grease or no grease, and/or you get a gorillas in boatyards with big spanners, and so there are plenty of scenarios where 5200 would work and 291 would fail, due to 5200's 3x strength. The fact that in your particular (perhaps lucky, perhaps careful) case you didn't need its strength doesn't change the general principle that anode bolts fall into the category where you want them really well glued (welded) on.

OK, instead of 5200, you can use sika 292. Fair cop Vic

 

[CHI]

JFM,s right. You must use 3m,s 5200.

Sikaflex not strong enough and will eventually fail. Best to do once and correctly

Cheers

keith
CHI