Skirting boards are the last pieces of fixed joinery to be painted in a room sequence. Treat skirting boards in the same way as any other woodwork: at some stage they will be so overloaded with coats of paint that you’ll need to strip it off and start at the bare wood.
You could use a hot-air stripper but take great care that hot-stripped paint does not land on flammable material. Chemical stripper is a lot safer – use a gel or paste so it adheres to the vertical skirting without dripping onto the floor and use a stripping knife to remove the ‘waste’. Follow the manufacturer’s instructions regarding application and neutralizing after-wards.
Much of the problem in painting skirting boards occurs when paintbrushes pick up bits of dirt and fluff from the floor, which is then deposited on the new paint. The best way to solve this problem is to sweep the floor thoroughly and then slide wide strips of cardboard under the skirting, which will also protect the floor from paint marks. Don’t use newspaper – it inevitably gets bunched up, torn and bits get stuck to the paint.