Just prior to construction, so that ALL surfaces, exposed or not, may be treated.
No, you should not apply urethane over deck stain. You should sand down the old deck stain first before applying any other type of finish.
terpentine should do the trick
You should seal it. Maybe stain it, too.
Yes.
Two or three weeks after the deck is built. You just have to wait for the lumber to dry out so the stain will penetrate the wood.
For a professional look to your stained wooden deck, make sure to clean your new deck boards thoroughly before applying any type of stain. New wood is usually covered in crushed wood grain called mill scale that is created during the milling process of the lumber. If you skip this step, the stain might not penetrate deep enough into the wood, so you might find yourself needing to re-apply stain again next year.
The first thing to try is plain water. Dawn dish washing soap is gentle but can help remove the stain. Another common product is dandruff shampoo. If none of these work, there is a product called Color Oops which will remove permanent hair coloring, so it should work on the deck stain.
can you stain over kool deck pool paint
Use the thinner of that paint and hope it doesn't affect the stain. There is no other way.
Can you put stain in polyurethane to darken the color?
Stain the deck.
70 degrees