I have an '89 Sierra Sportside, slightly customized, and have ran into a problem with the fibreglass box sides. I have been getting the truck undercoated annually for several years and this year the box began breaking out. It looks like rust bubbles but it's only on the fibreglass. I contacted the undercoating manufacturer and they told me that the product can soak through the fibreglass and bubble the paint. Has anyone had this problem before? I would like to be able to fix it without changing boxes as I have already had several body mods done to the box. Any help would be appreciated.