for what you need the sound absorbing paint will not work. I am assuming that the shop is empty? if there is nothing in there to absorb or block sound waves it will echo bad but when you put your boat and other stuff in there they will make it alot more quiet. The cheepest way to absord the sound would be to go to a cheep carpet store find a really cheep carpet and cut a big piece and hang it on the wall like a picture. carpet is usually around 12' wide, so maybe cut 8 pieces 12' x 12' and space them around the room, pick a decent color and it will give you some decoration too. If you want it to look better you can use base mounding or crown to trim around the carped and it will look more finished.