If you close the hatch to hard or even have a super loud stereo it will crack it. it is a ****ty way to fix a simple problem. Trust me i had a sub in a old kia i had and someone did this before i bought the car and guess what it was cracked about 2 days after i put the new sterio in. size of a 1/2" bolt. do this as a test make some bondo and let it cure put something from letting it stick to anything pick it up and see how easy it breaks. very easy. Ask any bodyman bondo is a finish product not made for filling a hole in a panel.
http://solutions.3m.com/wps/portal/3...beVBF19DZVHXgl
says right in 3m product sheet for dent goudge and cosmetic imperfection in steeel aluminum and fiberglass.
Has no structure without a base holding it up hence the welding.