Decks, Patios, Porches and Docks - Need help repairing railing posts

I'm working on a clients porch to repair their trex deck railing posts. The original builder did not secure the posts to the deck with lag bolts or hex bolts. They just nailed them, covered them, and finished the railings. Now, the nails are working themselves out. I want to do this the correct way by using lag bolts. Appearance isn't important to the homeowner, just functionality. Should I remove the coverings and take down all of the railings to do this correctly? Or can I just bolt right on top of the coverings? If the latter is the case, how do I go about it?
Any insight would be helpful. Thank you.

If thie deck was built properly, you should be able to get to the bottom portion of the wood post (below the deck) and bolt that to the joist and rim joist.

But it sounds like the rail posts were put on the outside of the rim joist. If that's the case, you've got bigger problems. The weight of that composite railing is (or soon will be) causing the rim joist to seperate from the rest of the framing of the deck.

If that's the situation, you have two choices. One is to disassemble the railing and move the posts to the inside of the rim joist, where you can bolt it into both a joist and the rim joist. The other is to disassemble the railing and add new wood posts tthat go all the way to a footing (which you'll have to pour) on the ground.