Rotting wood on porch

Would treat it with wood hardner cuprinol do one that's quite good,then fill with filltite two part filer sand down and paint. Two,three of coats of good quality paint/ stainer should get you an other few years out of it. good luck Alex.

I would first pour some wood preserver on that area, when dry use cuprinol wood filler, sand fill again if uneven, then paint using good quality undercoat and finish coat. Will last a few years. All the best.