hi, I have a small boxing in job to do, all ready done the box with plasterboard, just need to skim, have 2/3 of a bag of multi left from previous job, i only need 1/4 to 1/3 for box. Will this be ok to use as its 6 month old? been kept inside all the time. If it is ok, is there anything i can add to make the working time longer? any advisse welcome. thanks.