Bonding in acrylic portlights - recommended adhesive/sealant?

[Mark in Portishead]

The previous owner of my boat fitted a home built pilothouse over the companionway, sheathed in fibreglass and with acrylic/perspex portlights bonded onto some sort of white adhesive/sealant. There is no secondary, mechanical retention of the acrylic/perspex panels or trim such that the adhesive/sealant is visible through the acrylic/perspex . Apart from the fact that this sealant is becoming unsightly with black mould, during recent gales, one of the acrylic/perspex panels blew out and it is evident that there has been very little adhesion with the panel. Inspection of the other similar portlights on the pilothouse indicates that these too are debonding and likely to soon also detach.

I have read a lot about sealants/adhesives recently but have become confused by conflicting recommendations. Can the forum please recommend a suitable replacement adhesive/sealant that will adequately hold the acrylic/perspex panel and also not be so susceptible to mould? Or point me to a previous thread where this has been discussed?

Many thanks.

 

[Stemar]

This is how I'd do it:

Forget adhesive.

Paint a black - or whatever - strip round the edge of the window on the sealant side, and wide enough to hide the seal. Use butyl tape and interscrews or 3mm bolts with dome nuts. Don't over-tighten the fixings, or you'll squeeze the butyl out.

 

[fredrussell]

…or get in touch with one of the big players like Souda or Sika and ask them. Would have thought they would have a helpline or email address for technical queries.

 

[Mark in Portishead]

Thank you Stemar. That is a good solution for a 'proper' fix.

 

[PCUK]

Place a washer on each screw between the acrylic and boat. This prevents over tightening and squeezing the sealant out. It is the recognised correct way to do it.

 

[Mark in Portishead]

Place a washer on each screw between the acrylic and boat. This prevents over tightening and squeezing the sealant out. It is the recognised correct way to do it.

Useful tip for the future - thanks. In this case there is plenty of adhesive/sealant but it just hasn't bonded to the acrylic/perspex. Perhaps a poor product, perhaps residual grease on the acrylic/perspex - one or the other I guess.

 

[thinwater]

Actually, if you look at new boats, this is not how it is done. VHB tape and DOW 795, and no fasteners. I have done several boats this way. As for "not bonding," that is either the wrong product (the wrong silicone or any polyurethane without the right primer) or poor surface prep. Some silicons do not bond well to plastics, and polyurethanes loose there grip over time due to UV damamge (look closely and you will see it). DOW 795 is made for glazing (different product number in the UK).

installing frameless ports

 

[KAM]

I built a grp spray hood several years ago. The perspex windows were glued on. It's very simple and has worked perfectly. The key thing is to use a thin strip of foam rubber between the two surfaces. This allows a controlled adhesive thickness for differential expansion. There are plenty of details on line. You definitly don't need screws. I used Sticksall from Tool Station these modern adhesives are so good I don't think very expensive adhesives are necessary. It's easy enough to mask and paint the window border to protect the adhesive from UV and leave a smart appearance.

 

[mrangry]

I wouldnt use mechanical fixings as its just more hassle, the modern way is to bond with an adhesive sealant. I would use ct1, but have used sikaflex previously although their primers are very expensive. CT1 will bond to even a wet surface