What flexible gap filler to use outdoors

We are about to repaint the metal edging and railings around the veranda due to some rusting. Where the tiles meet the edge there is a slight gap between the steel and the tiles/concrete base. What is the best flexible sealant to use as it will be exposed to the elements. The previous owner has used something that has become brittle and cracked and is falling out allowing the water to run behind the metal plate and cause more rust. :o<O:p</O:p
Should the gap filler be applied before or after the top coat? We are removing all previous paint and treating the rust before undercoating with the rust kill paint. This will be followed with a suitable metal topcoat.

Any help or advise would be appreciated as we do not want to be doing this again for a while and would rather it be done properly the first time.

FWIW, I use sika flex pro, its polyurethane and stays flexible much better than silicon and is paintable, better than 303 also. Supply from CDA or someone, as bunnings charge $17 (almost twice the price at CDA) - not that it matters if your only buying one cartridge!

Thanks for that I will check out the sika flex sounds like it will do exactly what I want. I may need several cartridges though. The front verandah is almost the full length of the house and the back one is half the house length so probably looking at somewhere around 15m of edges to cover and fill the gap between the metal plate and concrete edge.

As for CDA I am not sure there is one in brissy but I will check out the price at bunnies and see how it compares to the $17 you mentioned.