Once you like the fit of the grill, now sand all areas to be adhered with 24 grit roloc disc and an air grinder.
Once you have done that clean all areas that were sanded with windex or the sem product, the plastic will give off oils when sanded and cause the problem plastic repair not to adhere.
Once cleaned tape off edges around grill opening to be molded to minimize the mess. Give yourself 2"s around the grill edge when taping.
Once taped then start filling the gaps with problem plastic repair adhesive, try and fill in the lines as much as possible. You will have to repeat this step a few times so do not worry if you do not fill all of the gaps at first.
Once you have run a bead around the grill openings spread the problem plastic repair with a bondo spreader, try and use the spreader to push the product into the openings.This product will set up or kick about 4 minutes.
You can sand it/grind it in about 5 minutes after applied.
If you have some big gaps use this SEM product, it will set up or kick in about 2 minutes so you do not have much time to work with it. Also the temperature outside will cause it activate quicker just bondo. This product does not flow much and sets real fast. So if you are having a hard time with the gaps not filling in use this.
