Ultimately, we can't tell you exactly how long you can leave butter out. We have no problem with leaving butter out to soften as long as it is well-covered. It usually doesn't take us more than a couple days to go through a stick anyway! We've never had rancid butter in that time.
For complete instructions, see my weekly column. Note: The vinyl melts in just a few minutes and is warm to the touch when you pull it from the 200-degree oven (don't use a hotter oven or the vinyl will start releasing toxins). You can form it by hand immediately, but use leather gloves if you're nervous. All beginners in Record Upcycling should try a bowl.
Final strength is reached later. Reactive polyurethane hot melts yield rubbery and plastic cured films. Curing hot melts have the advantage that they can be applied at very low temperatures, 125 to 250 o F (50 to 120Â°C). In contrast, standard hot melts are applied at higher temperatures 250 to 470 o F (120 to 240Â°C).