I know what he said and if done carefully there should be know worry. I hate trying to seal something pressurized for the outside with a silicone type sealer that is not meant to be for that purpose. High temps will create pressures up to the and past the cap rating pretty easy and (just because the cap pops at 15lbs. pressures can rise higher till bleed off is complete) and getting caught out on the road or in traffic with a hot engine may not be a whole lot of fun. Take a chance and do it right.