Some time ago I kinds "reinforced" underbody plastic protection covers, which at the end showed to be quite a good DIY approach to enhance underbody protection. Not as good as (expensive) steel or aluminium rock-slider plates, but still much better than factory plastic ones.
What I did was took off 2 plastic protection plates (under engine and transmission), measuring before the free space between plates and engine/tranny parts to see how much space is there.
An average free space was above 1 inch, so that's what I can deal with.
My plan was to:
- clean carefully both plastic plates
- straight-out and kinda repair cracks
- prime it with plastic primer (which I got included in one old set of plastic repair kit) so the fiberglass resin will stick to plastic (otherwise it does not)
- then put 3-4 layers of fiberglass reinforcement on inner/upper side
- and finally, fill the 2-inch gap with PU high-density foam
- reinstall with slightly longer screws to compensate for the additional thickness of fiberglass reinforcement
Sorry for the lack of step-by-step pictures. Here's just one in the middle of the process, with fiberglass layer finished:
After I chopped off excess fiberglass and re-drilled mounting holes, I sprayed a bottle of PU high-density fire-retardant foam on both plates.
THE TRICK with still wet PU foam was to cover it with non-sticking plastic foil, so I could press wet PU foam and shape it by hand to fill cover plate completely.
Then left over for 24 hours for PU foam to harden.
When mounting I took some time to cut excess with utility knife and the result is:
- approx. 1-inch thick protection plates
- better sound proofing
- better first-line-of-defense at least some protection against rocks and impacts, which proved to be quite good
