I would not worry too much if it is only a 1500PSI, usually electric, washer however I would not waste money getting one that weak.
I have a 2500PSI gas washer used primarily for boat bottoms, house roof, concrete driveway. Effective but nowhere near a good as a 3000 or a 5000PSI. Get yourself a nice soft brush, a decent length adjustable painters staff from Lowes or somewhere, a bucket of of soapy water & go at it.
Much more effective & safer for the graphics than a pressure washer. The only time mine has seen a pressure washer is in TOK, Alaska at a fuel station there where you can wash your RV after you buy fuel. Feels like a 3000PSI but I keep my distance & use it to get the chunky bits off then rinse after brush & soap.