Thread: De-laminating Gunwhales

Got any photos of said gunnels?

Howdy

I am not sure that the putty is a good choice - there might be another traditional material that works better.

There are few places for putty on a boat except for below the waterline to cover and finally waterproof caulked seams - it is the traditional way - but you need to put something in it (traditionally red lead - but some use antifoul I thnk - don't quote me) to prevent the worms from eating it all up - they like linseed!

Problem is that putty is eventually too brittle in air - boats of this construction move around a lot so something that goes brittle over time will crack up and then very effectively hold water against the wood.

In the current situation it sounds like the water can get in there but it can also drain and the gap is wide enough for air. This is not as bad as closing it up with water in the join.

I would recommend speaking to someone who actually knows about trad boats - there are lots that are happy to give advice, but not so many that actually know the correct more traditional way.

If you are looking at using Sikaflex anyhow - you could use it for the full joint. It will prevent water properly. The thing is that it will make it harder to get the bit off later - something that trad methods are quite good at allowing.

Personally I would use the Sika - but there are probably traditional ways of doing it.

If the gap is so wide that you would use a huge amount of sika - then there is probably some work required to fix or reduce the gap.

Michael

I do a fair amount of traditional construction repair, I'd use a polysulfide, my brand is 3M 101, but there are others. I like 3M 101 because it's not a particularly aggressive adhesive, but more of a sealant, that has some "stick-um" to it. I'd avoid a polyurethane as these can be quite aggressive adhesives and tend to yellow with UV.

If your surfaces aren't perfect, the traditional bedding compounds don't work well and this is a possible reason you're seeing some issues. The new man made stuff (polysulfide, polyurethanes, etc.) have tremendous gap filling and flexibility properties, which will solve your problem. Interestingly enough if you elect to use a polysulfide, it will cure and hold on moist lumber. Try that with an old oil based or lead bedding compound. Polyurethane needs to be under pressure (clamped joint) to hold on moist lumber. This is the reason I always use polysulfide under the waterline.

Phew! Glad you are around PAR!

MIK

Guys

Thanks for the feedback. Not 100% sure I understand why this type of sealant is being recommended and how it differs from the other sealans I know. Questions:
- Where in Aus could I buy the 3M 101 product?
- Alternately, is Sikaflex a polysulfide & would it work just as well?
- I am going to paint over it, does this make a difference?

Scott

Howdy Scott - I have replied to your direct email - no great news - but a link to 3M so you can ring them and find out who has the stuff.

If you cannot find it you can use Sikaflex - reading between the lines I get the feeling that PAR thinks the polysulphide is a bit better and also is a bit cheaper (am I understanding correctly) but he doesn't actually say not to use the Sika.

As far as yellowing goes - you can get UV stable versions of sika (more expensive and harder to find) or you can sand and paint it the same colour as the strake you are filling.

Best wishes
MIK

I haven't a lot of experience with Sikaflex products, only using it on a handful of occasions, but my data base says:

Sikaflex-291, is a polyurethane adhesive/sealant, that's reasonably aggressive in grip, with good flexural properties. It's considered an industry standard for many, though not as tough as 3M 5200.

Sikaflex-291 Lot, is about the same as regular 291, but dries slower. It's grip is weaker, but is slightly more flexible then 291.

Sikaflex-292, Is a thixotropic polyurethane, intended in bonding applications.

Sikaflex-292 UV, is similar to regular 292 with a UV inhibitor. The result is a slightly weaker bond and less flexibility.

Sikaflex-296, another polyurethane, but designed for bonding glass.

This is what I have from Sika Industries, for marine grade sealants and adhesives.

291 and 292 are commonly used. 291 as a sealant/adhesive and 292 as an adhesive with good sealing ability.

I couldn't comment on pricing, but suspect it's comparable with 3M.

The issue I have with polyurethanes is they need to be under pressure during the cure or they'll release from wet wood. This often is the case in use, where typically a gap will be filled with polyurethane and then smoothed over with a finger. If the wood gets soaked, this will fall out like a long rudder string. On the other hand, if you apply it to the garboard seam and then hang a plank, fastening down securely, then it cures under the pressure of the fasteners and will hold on to wet wood.

Polysulfide doesn't have these issues and it can be smeared on a clean dry surface, which it will remain stuck to, even if soaked.

Generally, polysulfides are what is used in deck seams (same reason) and is available in two part (the good stuff, but messy) and single part (in a tube or cartridge) not as messy. For the reasons stated previously, I'll only use polysulfide under the LWL, but will use the less expensive polyurethane above.

Sika also recommend one particular product for applications below the waterline. Probably of no importance if you are just gasketting the underside of a skin fitting - but if you are sealing plank seams - um - use the right one.

Also for critical high strength applications Sika has a primer which is brushed on before bonding. It costs the earth but it goes a long way.

For gasketting applications I WOULD NOT USE IT as you will find it difficult to get the fitting or window off the surface again. Even without the primer the adhesion is quite good - requireing serious work to get a window off for example - you often have to slide a razor blade under and cut the stuff.

MIK