how to get a key in dried sand and cement

[finch]

i did some internal rendering for a client who said they wanted sand and cement as the finish coat. so i rubbed it up to a smooth finish. They now (2weeks later) want it skimmed. so how can i get a good key in the sand and cement as i didnt devil float when the wall was wet.
any ideas?

 

[diyisfree]

If you have rubbed it up with a poly float then it will be rough enough.

If you have troweled it a little like skim then give it a coat of SBR and get it skimmed