My son (15) received two Mack brushes for pin striping for Christmas (without instruction) and is not really sure how to use them. He experimented yesterday but says the glue that holds the hairs on has disintegrated and now one brush is falling apart. As a parent, I don’t know if he did something wrong or if this is a manufacture defect and the brush needs to be returned. Can anyone help? Also, if someone can recommend a “beginner’s guide to pin striping” book, that would probably help too. Thanks.