Clear film-forming finishes (referred to generically as "varnishes") tend to fail by cracking and peeling in exposed exterior locations. A comparison of different finishes is included in our Technical Design Guide #13 Finishing Timber Externally. A copy can be downloaded from our website via this link https://www.woodsolutions.com.au/publications. It is correct that switching to an oil finish would require all the existing coating to be removed, since oils need to penetrate into the wood. Also any remaining patches of varnish would show through the oil. Paint will certainly provide the longest-lasting finish, and paint won't fail by cracking and peeling as long as it can bond to the substrate. While remaining patches of varnish won't show through paint they may present a jagged appearance at the edges, preventing a smooth, even coating and causing localised failure of the paint if the underlying varnish starts to lift. It would be far preferable to remove all the varnish if possible. You may obtain more detailed advice on this point from a paint company's advisory service.Exterior finish
Answered on 03-06-2021
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