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> Replacing Sonata Windows

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29th January 2011

by J Burgis.

Why would you do that?

Why? Because with age, they craze, discolour and crack around the fixing screws. This leads to leaks. If you are bold enough to sleep on-board and it rains then you will be well aware of this from the drips landing in your eye/ear.

My Sonata, Rhythm'n'Blue had all of the above. About 6 years ago I decided to deal with it but was unwilling to do all that drilling and screwing stuff. I rang Sika and asked them if they had a product for sticking Perspex windows to a fibreglass cabin top.

Yes was the reply. I was directed to use Sika 295 UV. They sent me a very comprehensive brochure / guide.

I have waited 6 years before recommending the method to anyone else. The windows are still there and appear as good as new. (I'll probably regret this when one pops-off next season). The boat has been wintered outside in lengthy sub-zero temperatures and has taken all the tropical heat the summers in the Highlands can throw at it.

And this is how it looks after 6 years.

So how would you do it?

  1. Take old windows off boat after removing all those bloody screws.
  2. Use old windows to form a template for new Perspex of same thickness.
  3. Order new Perspex cut to size from template. Unfortunately I can't recommend a supplier, as the guy I used is no longer in business. Alternatively order sheet Perspex and cut to size yourself. I've never done that for large pieces but found it tricky when I did relatively small stuff. I also enhanced the Perspex by having a small bevel formed on the upper edge of the sheet. The Perspex will normally come with a protective film on one side leave in place. Have the Perspex cut "handed" so that you can have the protection left in place on the outside of the job. You will be offered Perspex in varying shades of grey. I chose a mid density of grey. It appears stylishly dark from the outside but is clear from the inside. If you spend much time staring out of the windows of your Sonata then I suggest you get a life.
  4. Buy Sika 295 UV. Can be obtained from some chandlers. Is inordinately expensive. Comes in cartridges for use in a skeleton gun. I bought 3 cartridges, used 1 per side and had one spare (total cost 6 years ago app. £100). You also need Sika cleaner and primer. NB Sika 295 UV has a shelf life. Check that you have plenty of life left.
  5. Clean off the contact area. Use a Stanley blade carefully, white spirits will help. Abrade with coarse wet and dry. Take care to keep all abrasion within the area that will be covered. Otherwise you will mess up your nice gel coat.
  6. Buy spacer material. The instructions insist that the Sika is applied to a minimum thickness to allow flexing and differential thermal movement. I chose a self-adhesive neoprene strip obtainable from chandlers. It was 3mm thick x 20mm wide which I cut to half its width to give a continuous spacer 3mm thick x 10mm round the edge of the GRP opening
  7. Final clean the GRP with the Sika cleaner
  8. Mark the outer edge of the window on to the cabin side with pencil use the template. Add 4 mm to allow for the fact that the exposed edge of the Sika, after the window has been fixed, will require to be tooled off at approx. 45°.
  9. Mask round the area with masking tape but extend the area some distance with newspaper or similar. The Sika should squidge out as you fit the window and any dropped / spilled mastic sticks like shit on the proverbial. The Perspex film protection helps protect the window area.
  10. Stick on a couple of "stools" or temporary props. I reasoned that it would be difficult enough locating the window during the fixing process. To help I made these "stools" by sticking together 4 or 5 layers of self adhesive vinyl non slip flooring, obtainable from chandlers, resulting in a prop about 8-10 mm thick and about 30 mm high. And then sticking the "stool" at the bottom edge of the window. This provides a little prop for the Perspex and keeps it in its precise location.
  11. Apply Sika primer. Do so with extreme caution. Any drips on the GRP outside the window area will stain (irreversibly as far as I can tell).
  12. Apply Sika mastic to the entire area between the opening in the GRP and your scribed / masked line. Use a mastic gun to apply it but you are covering quite a wide area (rather than strip-application as in a bathroom). Apply it as evenly as possible to a thickness slightly greater than the neoprene spacer. You will require a spatula to spread it evenly.
  13. Place the Perspex in its correct location and gently press it back till you make contact with the spacer around the opening. If you have applied the mastic correctly it will squidge out along the full perimeter of the Perspex.
  14. I found it helpful to place two temporary battens across the Perspex tied back to the timber hatch runners at the top and the Genoa sheet track at the bottom. These were arranged to keep pressure on the Perspex so that I didn't have to keep finger pressure on for several hours of curing. Not much fun.
  15. Clean off the excess mastic that has squidged out. Tool-off the finished edge of the mastic with a small piece of wood tapered like a timber chisel. Remove the surrounding masking. Prise off the "stools" before doing this.
  16. Check inside the cabin. The spacer piece should prevent any mastic leakage inwards (I found) but you need to check and clean up if necessary.
  17. Leave for the prescribed curing period and remove battens.
  18. Now, one of the peculiarities of Sika 295 UV is that they insist that the mastic be protected from UV by a patent tape or paint. I checked with Sika and they were adamant.
  19. I painted the area of the new window from the opening in the GRP to the edge of the Perspex with a matt black paint. I used Halford's spray paint starting with their plastic primer followed by the matt black. This too has survived 6 years. And it looks rather fetching too. By scribing the Perspex protective film layer and removing the area over the mastic you will be protecting the open area of the Perspex. I used a Stanley knife but carefully. You don't want to mark or weaken the Perspex. Mask the cabin around the window. (You really need to remove the original masking at the time of fixing the window while the mastic is unset).

A few more tips

I ignored Sika's instructions on 2 matters:

  • They advise that the surface that the Perspex is to be fixed to has to be flat. The side of the Sonata cabin is slightly curved. It has worked for me so far. Anyway it is only a slight curvature.
  • They give a method of calculating the thickness of the mastic which came out to 4mm. I could only get a 3mm neoprene strip and I reasoned that there would only be a limited area at the minimum thickness of 3mm. So far so good.


Don't do this single-handed. I can vouch for the fact that it is a nightmare and you are liable to end up covered in black mastic. As will the boat.

Likewise, do not do this on a Sonata moored in the middle of the Cromarty Firth (or anywhere else). It will just add to the problems described above. I know.

See photo and sketches

I did this 6 years ago so I've gone on my best recollections.

Good luck

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Last updated 10:22 on 19 June 2024

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