If you’re painting over an old semi-gloss paint, check to see if it has become hard and brittle. Try to pick off a piece from an obscure area with your fingernail. If it chips, it probably needs a light sanding to improve the “tooth” for a new coat of paint.
When going to buy lumber, eliminate the guesswork by taking along the right gear. Take a good flashlight to be able to see wood in dark lumber sheds, plus a pocket knife to scrape rough-sawn boards to check grain. Also take along a good measuring tape, small square, pencil, notebook, and a pocket calculator if you will need to do much figuring.