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WHY CLEAR COATINGS FAIL

Have you ever wondered why varnished surfaces need constant attention or it breaks down readily, and whether there is a better way to achieve a long lasting clear finish for timber which is constantly exposed to the environment in exterior applications.  The following information is provided based on well founded research after years of listening to people that think they have the best finish, and some who have published misinformation as they do not have the ideal product range to achieve a long term durable finish.
This information is directed at the DIY and professional boat builder, but is equally applicable to the woodworker or home owner undertaking exterior clear finishes to a house or furniture.  The reference material used in developing this information was sourced from “Why Exterior Coatings Fail” by the US Forest Products Research Laboratory and testing by BoatCraft Pacific.  BoatCraft Pacific had samples of timber coated with three (3) coats of Bote-Cote Non-Yellowing Marine Epoxy resin and then three (3) coats of clear AQUACOTE Topcoat placed on test panels at Townsville for the equivalent of seven (7) years with no maintenance and with minimal loss of gloss evident at the end of the test.
The failure of coatings is dependent on a few things, most commonly:

  • Exposure to moisture. and
  • Exposure to Ultra Violet (UV) rays as the primary causes.
  • Type of coating applied.
  • Inadequate / Incorrect preparation.
  • Sharp corners / inadequate radius.
  • Coating damage and joint cracks.

Proof is in the Finish

Figures 1 & 2 are two interesting photographs of brightwork in the cockpit of the 70 foot yacht, El Karim.  Additional photographs of the brightwork can be seen at http://elkarim.com/photo-gallery.php.  The surfaces were prepared with three (3) coats of Bote-Cote with NY Hardener, followed by three (3) coats of Aquacote Clear approximately six (6) years ago.  Since then the boat has been across the Tasman 4 times, Fiji, Vanuatu, New Caledonia several times and now back in Australia.  No maintenance has been carried out to this bright work.
Figure 1 showing the winch is very telling as it shows where the sheet has rubbed over the bright work and damaged it, which allowed moisture and UV rays to penetrate.  You can see how the water is moving further under the coating, causing oxidation and damage to the timber and the de-lamination of the coating is spreading.  The damage is absolutely confined to the locality of the failure of the protective film, which allowed moisture to enter.  It is also pretty apparent that the degree of sun access to the area under the film which is failing is not a factor.

Damaged

Figure 1
El Karim Clear system localised damage
The other photograph is a spare winch pad which clearly demonstrates just how well the system holds up after 6 years.  It is perfect, showing no signs of failure where the timber has remained sealed and therefore dry.

good cond. after six years


Figure 2
– Spare Winch Pad in good condition after six years exposure using Clear System
In summary, if you enjoy sanding and painting use varnish or oil for exterior timber protection.  If you want to spend less time sanding or are time shy use the clear system for a long lasting Clear finish.

One last piece of advice, do a section you can complete in two or three days.  Do not try and do the entire boat in one go as it will become a major chore. Further the timber needs to be freshly sanded to ensure good adhesion of Bote Cote to timber.

For further information on why coatings fail refer to Cruising Helmsman September 2011 articles titled "Finishing the Job"- www.mysailing.com.au or our technical information sheet at Why Coatings Fail

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BIAMember BEVERLY HILLS NSW  2209
(02) 9533 5470
Bote.Cote@optusnet.com.au
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