Don't put fibreglass on the front of your kit.
Take the kit off and glass the back of it then when it's dry, sand back the crack on the front prefferably by hand and definately not with a grinder.
When you have a clean and smoothish surface, use body filler to fill in the cracks than sand with a block or by hand depending on if the area is curved or not.
Use about 3 or 4 layers of glass so that the repair cannot flex and that way the body filler won't crack once it's painted.
When you read the instructions for mixing the resin, mix it with the minimum ammount of hardner required. It may take a tiny bit longer to dry but at least it won't turn all lumpy before you get a chance to use it.