I had a re-occuring crack in my plaster wall. On the advice of the clerk at my local home depot i bought an elastic custom patching compound .As is my nature I did not read the directions carefully and spread the compound liberaly not worrying to much about how smooth it was thinking I would go back and sand it smooth the next day .When I tried to sand the patch even 60 grit sand paper wouldn't touch it .When I read the directions carefully it said this product was not made to be sanded .Any suggestions as to how I can level and smooth this product? Thanks for your help in advance .