It would look pretty but not very practical, considering that the heat of the exhaust would probably ruin it in a few minutes of use. The only actual color I've seen used on headers is flat white, which I suppose could easily be repainted to show them off. By the way, do you understand that most candy apple colors require you to have a highly reflective metal base, before the transparent red is applied? ref: glue wait time: If you applied enough glue to the mating surfaces so that it oozes out the side, I would check that exposed glue for "done-ness". In other words, once it's cured so that it can be sanded without spreading it out further, you know it's ready. You'll get a "feel" for this over time.