According to dpb :
aspasia wrote:
My new side porch has treads and handrail of Trex,
a composite wood.

The Trex website says to wait 12-16 weeks for it
to "fade" before staining to get best results.

Any experience here? Really need such a long wait?

No, the manufacturer obviously don't know what they're talking about
since it's only their product. Go right ahead.

Geez!!

Heh. ;-)

I'd also wait the recommended time. The reason being that "fresh" plastic
often has a chemical film (eg: plasticizer) that will wreck paint adhesion.
Epoxy does too (West calls it "bloom") if you're not careful [+].

Letting it weather outside for 12-16 weeks will allow time for the
film to be eroded/washed away. Abrasion from foot traffic will also
cause microscratches that will also help the paint adhere.

You may be able to shorten the time by using a pressure washer and strong
cleaning agents, but I'd still give it plenty of time.

[+] If you allow West system epoxy to fully cure before recoating or
priming/painting, you're supposed to give it a good scrub with soapy water
(and a fine scuff sanding if you're putting on more epoxy). See the West
handbook for more details.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.