How to patch under skirting.

you need to use bounding or hardwall make use u use some pva before

I’d check the cause of the damp before doing any repairs. Have a look at the external brickwork, pointing and ground levels, and check for gaps where slugs might sneak in. Once the source of moisture is sorted and the wall has dried out, remove any loose plaster and make good with a suitable repair mortar or plaster. Don’t fit the new skirting until the area is completely dry.