Water Sealer for Garden Shed

Recently got a shed frm Ballyfree. They told us to use a shed preservative and then a water sealer a few weeks later.
Have looked in Woodies, Atlantic etc, but any of the water sealers I have seen are for brick and specifically say "Not for wood".
Anyone know what I should be buying, and where to get it?

Thanks,
S

My wooden garden-shed is over 25 years old and still in good condition. All I've done is put a coat of Cuprinol on it every other year and I've replaced the roof-felt once.