You can use a good quality powdered filler which doesn't shrink or crack and is easy to sand flush. The drawback with this method is that the skirting will obviously expand and contract with differing conditions and possibly cause a crack where the filler meets the timber.
Before filling with anything, you need to be sure that there is not a void behind the skirting as you will end up with all the filler disappearing into it.
In some buildings a small gap can aid airflow so that may be worth considering.
This is why I was thinking something along the lines of gripfill, as I don't think it will shrink. Even a powered filler I have seen shrink and will probably need a second layer to finish it off.
There is nothing I can do about the void behind the skirting short of removing it, filling it and putting it back again. But for that I might as well replace all the skirting anyway, so it's a moot point really.
Ideally I'm looking for a flexible, non-shrinking gap filler. But even manufacturers claims about non-shrinkage I take with a pinch of salt.
A grab adhesive/filler will never allow you to get the smooth finish you will achieve with a proprietary filler.
I'm not saying you can't try it, but I personally wouldn't expect it to work well.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local