Unless treated with a wood oil, it's always better to remove any previous wood treatments and take the item back to clean, bare wood.
Wood oils are a good option for exterior wood as they penetrate into the wood grain and protect the wood from moisture and weathering. Oils are very easy to apply and maintain and will never crack, flake or peel over time. For Iroko, which is an oily tropical Hard wood, not all exterior wood oils would be suitable. Our recommendation for Iroko gates would be Osmo Decking Oil. This is because it is a thinner oil than others and more suited to exotic hardwoods. Although it is clear, it will darken the wood to some degree in the same way that water would. To get an indication of how the gates will look if oiled, once sanded back, wipe over a section with a clean sponge and water to dampen the wood. How this looks when damp is a good indication of how it will look ones oiled.
Alternatively, Sikkens Cetol HLS Plus followed by Sikkens Cetol Filter 7 Plus could be an option. These are coloured, varnish type finishes that coat the surface of the wood. The pigments in these products act as UV filters to provide the wood additional protection from the greying effects of UV damage.
Both of these options will weather and require maintenance from time to time. It's always difficult to give a maintenance time frame as it really is dependent on how much wind, rain and sun the gates get. Saying this, it's normally easy to tell as the finish will start to loose some of its lustre and will start to look tired and perhaps a little dull.
As Iroko is a naturally oily wood, it should be wiped down with Methylated Spirits, after sanding and before applying either of the above products. This is to remove any surface oil or grease that could cause adhesion problems and especially so with Sikkens products.
Before starting any project we always recommend doing a test area to assess product suitability and final finish.