A few turns of masking tape work fine, assuming you're screwing rather than nailing, that is. Just hold the rail end in place too while putting the first screw in. My earlier suggestion to fix brackets to posts first follows having done it many times with mortice adapators into concrete posts, where you don't have the luxury of choice.
As the reinforced concrete posts of the original fence had managed to last nearly 70 years, I reckoned that was a good material for the replacements. I also put in concrete gravel boards along the bottom of the fence. If you do concrete wooden posts in, make sure that the post is bedded slightly into a layer of gravel before you pour the concrete. That avoids a water trap at the bottom of the post. Where I have used wooden posts within the garden, I used oak, which was not a lot dearer than pressure treated softwood and it still looks like new seven years on.
Colin Bignell
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