Just don't use silicone sealant on your engine!
No seriously, the black brush on stuff is jolly good. My car had nothing else on the underside for 30 odd years (i.e. on bare steel) and it only rusted when the coating started coming away - it does eventually dry out and fall off if not re-coated. I agree with Hedd though, paint then apply over the top. Then take a look underneath once a year or so and address any signs of failure.
Check you don't need to weld anything before you apply it rather than after!
No seriously, the black brush on stuff is jolly good. My car had nothing else on the underside for 30 odd years (i.e. on bare steel) and it only rusted when the coating started coming away - it does eventually dry out and fall off if not re-coated. I agree with Hedd though, paint then apply over the top. Then take a look underneath once a year or so and address any signs of failure.
Check you don't need to weld anything before you apply it rather than after!