guys, i painted my plank with polyurethane
and now its curved
how come ? 

jilaka, the seller said its dried wood
anything i can do ?
been putting it level on the ground but still warped

if its not to big piece, you can put in the back seat of your car, since we have hot day time. after a week alot of the moisture would have left the wood

if the wood want to warp whether u dry it within 1 month or 1year also it will happen, as u said about your tembusu after 10year still warp..
so your recommended method still will warp also.. and need to wait 10year to find out!

anyway, after it  dry then we will reshape and mill it to precise dimension and remove all the warp and twist in the process.

so the important part is learning to mill the timber to exact dimensions after the timber has warped.

for example, pic below. 6feet american white oak timber and after process or milling, warpage is max 0.5mm. thats tight dimensions and ready to use. (previous 3-4mm warpage at high points, no issue, all taken out of the timber)