So, I did not realize that Frontrunner racks have plastic spacers at the feet. Tried using the butyl tape and washers anyway and it was not good. The spacers don’t press against the washers evenly. Had to take the siderails off again and remove all the butyl tape and use silicone the next time...
Thanks for all the work and research you put into it. Going to be using this method this weekend to install my Frontrunner rack.
For anyone that is using this method that also had a hard time paying $36 for 12 washers, I found almost the same thing on eBay for $10 shipped. It is not marine...