My folks chip out any loose concrete, acid etch it and treat any rusty steel
. And then after painting on penetrating epoxy
, repair it with epoxy
grout. They had a few small voids where fixed many years ago and haven't given any further problems. We punched a hole in the side once on a very dodgy, exposed wharf when some friendly chaps moved the boat while we were away and didn't shift any fenders. We repaired it with normal epoxy, not as good and some small hairline cracks around the join to the concrete have shown up, but it's still going strong after 25 hard years of sailing.
There are some great products on the market now for ferro
. Just make sure the problem isn't too widespread, if its just a few patches it's likely a small area of poor plastering and easily fixed. If its all over the place its likely a poor build, bad mix or cure and the more you dig the worse it will get.
Some photos would help.