How to repair broken render

I'm currently renovating my port and I've stripped off all of the paint from the below block. However some of render (I think) has come off previously. Can I just mix up some sand/cement (sharp sand?) and trowel it in, or do I need some render repair or similar?

I would either take it all off and start again, or if you are going to paint it just knock off all the loose bits and render it. You will probably need to put some batten up.
You can buy bags of ready mixed cement, just add water.

You'll end up with two different colours if you patch, hence I said about painting. I can't see how thick the render is, you might have to do several layers to build it up.
Clean off all the mold with some bleach.

You can see there is cracks all over that. Chip off the loose bits. Slap some PVA mixed wi§th water before troweling. Use two coats one scratch, let that dry then scratch it the top smooth coat.