I don't know how your accounting patern really is. Accounting is done as per the requirement of reports you need. Lemme ask some more question. Do you need to know the profit of each contract ?
Then the next point to be considered is materiality and occurance of the expense. If it occupies a major expenses of the contract my opinion is that it can be accounted as painting expenses itself. But when we are reading financials it may not be clear and can lead to confusion. So what you need to do is to add something between painting like solar like panel painting expenses or something like that. You have not mentioned where does the paint apply.