bought 8 yr old house with Trex decking & cedar posts/rails/spindles that previous owner painted with latex. Top boards (approx 8 inches wide by 4-5 ft long by 2 inches) had paint flaking off. I sanded smooth, primed with Sherwin Williams A100 and top coated with Duration. Paint bubbled bubbled (air pockets). Stripped again, this time I used exterior spackle and skim coated / sanded in case there was something on the wood ... same story - bubbled worse. So, now I am thinking sand one more time, try exterior KILZ and see if the primer will hold. Duration on spindles/bottom rail looks great. Top board/rail is the issue. I was also thinking about putting some sort of gloss white vinyl cover on the top rail and averting this issue altogether! Can't locate anyone who does this. Any advice??? Thanks!!!
Although I've never used it, Melamine paint may be an answer. Wait for the pros to guide you on this problem though. I have read about the Melamine paint (which is basically liquid plastic) and according to info can be applied to any finish and it will dry to a hard, smooth finish like plastic. It might be an option for you.