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Glue/sealant question

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Graham W:
I'm in the process of replacing my depth transducer.  It used to be standard on BayRaiders to attach a transom-mounted type of transducer just ahead of the centreboard slot to a specially shaped block of wood that fixes inside the very front of the slot.  Photos of the sloping fixing position and of the block of wood with attached transducer below.  Note the screws into bare GRP!

For various reasons to do with lack of decent access to fixings on this particular transducer, the only way of installing it is first to attach it to the piece of wood and then to bond the wood to the GRP surface inside the centreboard slot.

The question is, what sort of bond/sealant/glue is it best to use to ensure a good strong bond between the piece of wood and the GRP, given that the two of them are going to be exposed to seawater?  It wasn't that difficult to unstick the wood from how it was originally installed by the yard.  Obviously I will need to clean up the two surfaces before applying anything.  Looking at the sealant remaining on the piece of wood, it seems to be a mixture of some cream-coloured stuff and some black sealant, which may or may not be Sikaflex.

All suggestions gratefully received.

garethrow:
Hmm, have you got any way of clamping / applying pressure to the block whilst it is glueing in place Graham? If so two choices spring to mind: a) epoxy it in place as a fairly permenant / bomb proof solution, or b) use the stuff that the yard use which is a black bostick marine consruction adhesive: https://www.sealantsandtoolsdirect.co.uk/bostik-simson-msr-marine-construction-adhesive-sealant-black

If you can't get a way of applying pressure during the bonding process - may have to resort to some screws as well?

Regards

Gareth Rowlands
GRP BR20 Halen Y Mor

Graham W:
Thanks Gareth.  I think epoxy is a bit too permanent so I’ll try the Bostik.

I’m thinking of temporarily attaching a jig to the underside of the piece of wood that will allow me to apply pressure by bracing it against the outside of the hull.  Once the adhesive has cured, I can unscrew the jig and hopefully everything will stay in place. 

Sea Simon:
"Glue"
Sikaflex 291 is excellent for this sort of thing, and widely available too.
You can sometimes  get it in less wasteful (and cheaper!) smaller tubes, but check expiry dates.
I've had mixed results  with CT1 construction adhesive  outdoors/underwater. But otherwise an excellent (relatively) economical alternative.

Clamping.
Try wedges etc..I'm  sure you've done it all before?
I have found these mini air bag/air wedges really useful for this sort of clamping job (in holes/crevices).
https://www.screwfix.com/p/winbag-inflatable-air-wedge-160mm-160mm-x-160mm/1363k
Mine were from Aldi, and about 1/3 the price!

Graham W:
Thanks Simon.

Does anyone know if there is much to choose (apart from smaller sizes available) between the Bostik and the Sikaflex?  And can Bostik shelf life be extended, as it can with Sikaflex, by putting it in the deep freeze?

I could get much better access to the transducer installation if I was prepared to remove the centreboard and/or the centreboard case capping.  However both are big jobs, so some sort of temporary jig seems to be the easiest way of applying pressure to the installation while the sealant cures.

If you look at the first photo above, I think an inflatable wedge would be in mortal danger from all those bare screws!

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