Darrel obviously knows what he's talking about and he gave you some excellent information.
I personally would approach that repair from a little different angle. First thing to do is wash down that whole area on the inside and out side with soap and water to remove any dirt etc. Dry it off good then wash it down really well with a wax and grease remover to remove any contaminants on the inside and the outside of that bumper area. Next get some 36 or 40 grit sandpaper and hand sand the INSIDE area really well all the way around the hole and extend the sanding to at least another 6 inchs around the hole. Leave that area for now and concentrate on the outside of the hole. I don't know if you have any air tools, air compressor, air sanders, grinders etc. I'm going to assume for these instructions that you don't to make things simpler. Get some 80 grit sandpaper and sand all the way around the damage/hole extending out at least 6" around the hole. ALL the paint and primer must be removed and the bare fiberglass needs to be roughed up well with the 80 grit paper. Make a small wood sanding block (out of a paint stick or whatever works) and chamfer the edges of the hole so that the sanded edge tapers back some. At this point the outside is ready so you go back to the inside/backside of the bumper. Get a chunk of scrap plain old white styrofoam like they use for packing/shipping. Cut and shape with a razor knife and sandpaper the piece of styrofoam so that it loosely fits the contour of the shape of the missing part of the bumper from the inside/backside of the bumper. You're not looking for a perfect fit, just something relatively close. Next get some Saran Wrap and wrap/cover the styrofoam "buck" you made with a layer of that saran wrap. Push the saran wrap covered styrofoam "buck' you made up into the hole in the bumper and tape that styrofoam/saran wrap in place from the inside so it can't move around (it has to be tight up against the bumper. Go to the automotive paint store and buy a quart of fiberglass resin along with its tube/vial of liquid hardener and a small package of fiberglass MATT (not cloth). Get some disposable gloves and some cheap disposable paint brushes around an inch to an inch and a half wide. Pick up several mixing cups at the paint store while you're at it and a handful of wood disposable mixing sticks. When you get back to your shop get out your trusty scissors and start cutting sections of the fiberglass matt that you will use to fill in the hole in bumper. The first patch you cut should be at least an inch bigger around than the hole in the bumper. You'll need two more patches of the fiberglass matt with each being about a half inch longer than the previous one. (you want the outer edges to taper some so that you don't have three equal size patch's stacked up at the edges.) Read the direction close on the glass resin can and pour about an inch and half of resin in the mixing cup you bought. You should be wearing a pair of disposable gloves at this point. Lay your first glass matt patch you made on some clean paper or cardboard and using one of the disposable brushes you bought dip in the mixed fiberglass resin mixing cup and apply some of the mixed resin to the first matt patch laying on the cardboard. You need to saturate that glass matt with the fiberglass resin. Too much resin is much better than not enough. Next take your brush and mixed resin and brush the resin in all around the hole in the bumper (at least 3 inches past all the way around the hole.) Lay your first saturated patch on the hole. You have have to make some slits in the patch with scissors to get the matt to bend around tight corners). Go on to your second patch the same as the first and then on to the third patch the same as the other two. If your shop/garage isn't heated and the area is cold then you need to get an electric heat pointed at the repair spot to keep that repair spot warm (not hot). Let it sit and go eat lunch, drink a beer or a pop and take a break for at least an hour. After that check with your finger to make sure the new fiberglass is hard. It has to be hard like regular fiberglass to move on to the next step. Remove the styrofoam "buck" and saran wrap from the back side of what was once a hole. Get out your 36/40 grit sandpaper and sand that area again on the inside so that the whole inside area around the repair is rough with a lot of coarse sand scratches. Using an air gun (if you have one) blow off any dust buildup or wipe it off with a clean brush if you don't have air. Cut three more patches of the fiberglass matt that need to go past the outer edges of "hole" by at least a couple inches all the way around. Do the same thing you did on the inside that you did on the outside with the resin and the brush. Again you might have to cut some slits in the glass matt to go around tight corners. Cut smaller patches if you have to in those areas to make sure everything is covered and will be solid when the repairs are done.
Go drink another beer or a pop and let that set for another hour or until the matt/resin are hard to the touch. For all effective purposes the inside/backside of the bumper repair is done at this point. You need to concentrate on the outside/front side of the bumper repair with 36/40 grit followed by 80 grit. This is where things get relatively labor intensive if you don't have grinders and sanders. If you don't have any power tools then buy or make a sanding block and start leveling out the repair area. It sounds kind of silly buy sand off everything that doesn't look like the shape of the finished bumper. You will end up using some polyester spot putty to fill in the surface imperfections and air bubble holes in the fiberglass.
Your repair is done at this point and you get into a whole new segment of painting the repair area. That is a whole other story.
Something that needs to be pointed out -- there are several different ways using different materials of doing this fiberglass repair. There is not a single right way or wrong way of doing this as long at the finished product is a quality repair that will last as long or longer than the rest of the bumper.
Anything goes in the world of fiberglass and plastic
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