Hi - I have been lurking on this list for quite a while and find it a great source of both information and ideas. Unfortunately whilst I love working with metal my area of expertise is computer network design so I usually cant offer much advice. My current project is to repair my Nissan Xtrail's bumper bar - I backed into a steel post - the paint cracked but the bumper appeared otherwise undamaged. I have sanded the cracked paint with DA sander 320 grit then wet sanded with 400. I applied some primer for plastic (which seemed very 'runny'), then applied acrylic primer surfacer and all appeared OK. I then applied a colour coat of acrylic laquer about 4 hours later and before the colour coat had dried it cracked around the edges of where I had sanded. Strangely on the top part of the bumper there is no cracking of the paint. I am sure it is a simple problem - does anyone have any ideas?