Whats the best wood preservative for deck joists and posts?

[Gary111]

Hi, I have a 20 year old deck which was pressure treated copper arsenic I think

I am trying to preserve it as long as possible, I want to treat the joists etc with something strong.

What is the best treatment for that please?

I guess these products are basically insecticides?
Thanks

 

[^woody^]

The existing preservative will still be preserving. If you must, any preservative will do, but only in the end grain of the timber will be worthwhile. All tops and sides should be oiled or coated.