Seam sealer may be useful to you for finishing/hiding repairs..comes brushable in a tin or a cartridges for finishing joints that you use in a standard silicone/sealer gun.It's what the manufacturers use where they join the cars together.Very strong & rubbery...When applied you can wet your finger to smooth it out or with the canned stuff you can dip the brush in thinners or petrol to smooth.Not too much thinner or it will craze up when it dries..
Underseal products stay sticky,ok for under cars & arches.I prefer stonechip for under carpets & visible areas as it dries hard & can be flatted & painted over if needed.Great for sills,puts a hard rubberry coating under the paint..