How to best preserve the wooden planters for ornamental plants

kaboo

Hi everyone,
I have been improving our garden since moving in our first home 2 years ago. With the space also started growing some fruits and vegetables. For these I got some good quality, almost new clean palettes (heat treated and used to supply research lab equipment). Ok with them to rot but dont want to use any wood preservative. I do line them with empty compost bags and they have survived a year, hopefully I will get another year out of them.

I am also looking to make some planters for ornamental plants to put in the front driveway. I have got some pressure treated decking boards to build the planters. I want these to last so can use a chemical to preserve the wood as I will not be growing food in them. Plan is to wash the boards, treat them with a preservative, seal the inside edges with silicone caulk, apply a top coat of decking oil, put a liner and plant! 

But I dont know which preservative to treat the boards? Then which top-coat to use? 
Thank you

Nobbie1967

The decking should be fine with its existing pressure treatment. Nothing you brush on is going to make much difference. You will need to treat any cut ends as this will expose untreated wood. This is designed for the job.

https://www.wickes.co.uk/Barrettine-Timber-Cut-End-Preserver---Clear---1L/p/254898

wort

A good tip on Gardeners world was to put a metal “angle iron” sorry I can’t remember the word he used basically a metal 90 degree L shape on each corner, to strengthen as the inner timbers rot.