The wife just got a new Chrysler 200. She likes the Factory fitted splash guards. They mount using 2 existing wheel lip screws but you need to drill a hole in the bottom of the body for a 3rd mounting screw. I have heard 3 ways to prevent rust starting around the drilled hole. 1.Paint the hole with touch up paint & insert screw. 2.Paint the hole with touch up paint & apply silicone liberally to seal everything. 3.Paint the hole with touch up paint & apply loctite liberally to seal everything. I intend to replace the 3rd screw with stainless steel. Which of the 3 is best or, is there a 4th way?

I usually have some of the wax based Eastwood rust preventative on hand. Its the stuff you spray in hidden areas like rockers.

I dip the screw threads in the rust preventative then install. It sets up to a semi solid wax and seals it all up. Easy to remove also.

there might be an adhesive silicone that you can use instead of drilling.

a body guy might be able to chime in.

Stainless...

Dip the heads in a little wax...

Option 4: find something that doesnt require drilling holes in a brand new car. Beyond BS that chrysler cant come up with something that requires doing permanent damage.

Dunk the screw in grease before you put it in the hole and it shouldn't rust. If you're worried drill the hole, put some touch up paint in it and then put the screw in.
Some mudflaps come with screws that have sealer stuff on the threads.