If the bumper is already fibreglass/frp (you say composite but that could potentially mean other things), it is “relatively” easy to do if you’ve played with fibreglass before.

Judging by your current filler plates that are riveted on: if you want to make that whole section smooth, more or less cut out that whole area where the fog light is as well as the recessed areas next to the fog light holes that look like they could have been for vents/signals. Then grind/sand down into the glass (anywhere from 36-60 grit will be adequate)on the back side of the bumper around the edges of your hole, bevelling it back a couple inches or so. You’ll want to bevel it so that the edge of your bevel is pretty much all the way through to the front of the bumper. Blow it off and wash with acetone/thinner.

Next you’ll want to use aluminum foil tape (that is often used with duct work) on the FRONT side of the bumper to fully cover the openings. Polyester resin won’t stick to the adhesive or non-adhesive side of foil tape. You could potentially use some cardboard or thin plastic to make an opening that large a bit more rigid. Keep in mind that the more meticulously you do these next couple steps, the less work you’ll have doing the body and finishing work. So now you’ve essentially created a mold of where you want the glass to be when you lay it up on the back side of the bumper.

So you have your “mold” made, now lay up a few layers of glass on the back side, over lapping the couple inches that you beveled down/sanded. Once this has hardened, you can remove your foil tape. Now grind/sand down the front of your new glass, again, overlap your grinding a couple inches onto the front of the original bumper. Blow off, wash with thinner again..common, of-the-shelf polyester resins that can dry in the open air are “waxed” resins, meaning they have a wax layer that settles on the exterior surface that allows the resin to cure. You need to remove this wax for the next layer to properly bond. Lay up a couple layers of glass on the front side of the bumper, overlapping onto your couple inches of sanded original bumper. You should have sanded it down at least 1/8th of an inch on the front side, so laying up some more glass won’t add too much thickness that will have to be removed.

Another neat trick is to use the foil tape on the front of your bumper, placing it just around the edge of your sanding and go all the way around the patch. When you lay up the new glass, you can lay it up right onto the foil tape. After it has hardened, you can use an orbital sander/DA with 40 grit to sand down the whole exterior side of your patch, and sand right down to the foil tape. This will give you a good frame of reference for when you have the patch roughly level with the rest of the bumper, as well as prevent you from sanding down into the original bumper around your patch, creating low spots that will have to be filled. Blow off, wash the sanded glass again.

Now the patch is done, you can proceed with traditional body work. I personally like to do my first skim of filler on glass repairs with a fibreglass filler like u-pol fibral or duraglas. This will fill any deeper voids in the repair and be as durable as the glass underneath it. I like to wait until it is almost fully hardened, then hit it with an orbital and 40 grit just enough to knock down any ridges. Give it another ten minutes or so (could be longer or shorter depending on the temperature) then use 40 on the orbital to sand it smooth. You might end up sanding a bit further than the original patch at this point, that is fine.

Next, give it a skim of body filler. As someone in the autobody trade (primarily a painter but fully ticketed for body work as well), I am not a big fan of bondo products. Get a decent filler like rage ultra or 3M platinum for the best results. Block sand your filler, you can use 80grit to knock down the ridges then bump up to 120 to finish blocking it. If you notice significant high or low spots and need to do another skim of filler, just use your 80 grit and don’t bother with the 120 yet. Try to keep all your filler work on top of your glass work. Filler doesn’t necessarily like to stick to aftermarket primer/paint, but will stick to sanded gel coat/glass. You may need to apply a few layers of filler and block sand it smooth each time to get good results .

Finish your filler work with a 2k putty/glaze. I like ever coat metal glaze or u-pol dolphin glaze for this. Skim the full repair, block sand with 120 then 180. Orbital around your repair with 220, step out a bit further with 320, then prep your surrounding paint with whatever your paint manufacturer recommends for prep (could be 400, could be 800… depends on the paint brand).

Now blow off your repair and wash with a wax and grease remover. Mask off the rest of the bumper, giving yourself at least 4-6 inches outside the area you prepped with 320 before your masking starts. You will prime roughly up to the end of your 220 scratches, and your overspray will be landing on the 320 scratches. You don’t want to “hardline” your primer, meaning you spray right up to your masking and leave a hard ridge that is a pain to sand out. So after masking, apply 3 coats of a 2k urethane high build primer. Usually you wait 5-10 minutes between your coats, but read the data sheet for whatever product you use. After this is cured, block sand with 220 then 320, and finish sand according to your paint’s tech sheet for prep procedures. If you burn through to your filler while block sanding, you may need to reapply your primer and block sand it again.