What's the best filler to seal gaps in floor before polyurethane/waterproofing paint?

[Handtrowel]

Hi, just want to know if there's a better alternative than using tubes after tube of caulk to seal up gaps around the edge of a hardboard floor before putting some heavy duty floor paint down.

I've seen someone using a flexible trowel to form a kind of coving-shaped bridge between the wall and the floor, before applying tanking paint, but I've got no idea what it was that he was trowelling in.

Any advice?

Thanks