On 21 Jul, 20:38, Pete C wrote:
On Jul 21, 4:41*pm, (A.Lee) wrote:

Just basic decking boards, probably 4 years old. I sanded down to get it
smooth.
Painted then with this :
http://www.cuprinol.co.uk/products/total_decking_overview.html

Or something very similar. It was £25 anyway, so not the cheapest stuff
available.
It got 2 coats over 2 days.
3 months later, it started to peel off.
I'd presume the wood was too damp for it to be used, though i certainly
wouldnt have painted it if the wood was wet, though maybe the surface
was dry, and under was damp?

Maybe the deck boards were pressure treated with a wax based
preservative, which a water based coating wouldn't stick to very well.
Sanding the boards may have exposed fresh wax too.

For a strong dark colour I'd use Sikkens Cetol or Hicksons Restol, I'd
also try stripping any wax/oil from the surface with a strong solution
of 'oxygen bleach' followed by lots of rinsing.

Alternatively a decking oil should be more compatible with any wax
based treatment already in the wood.

cheers,
Pete.

Please define "basic" decking boards - obtain data from supplier as to
what treatment (if any)
pressure
os
aquavac

what species?
etc

Otherwise I cannot help you
Chris G