Scrape it first to get the bulk of the stuff then a good degreaser then some dwell time then hot water wash.
Have you checked the temp of the water coming out of the burner or at the end of the hose with a thermometer? Most of the time the water temp is not the same as the setting that is on the dial on the heat control.
This has been debated over and over and over and over and then proven. Most machines will give you 120 to 140 degrees heat rise over ambient temperature but that is what is coming out of the coil, not at the end of the hose so the longer the hose the more it will start to cool and lose heat.
Don't believe what the manufacturer's claim, it has been proven several times that you will not get the temp that is on the dial at the end of your hose, you might get close to that coming out of the boiler but you do not use 15' hoses to use that heat, most people have 100' to 300' hoses so the water will cool down some by the time it comes out of that hose.
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Superior Power Washing
Chris Chappell
361-853-2513
prostaff@superiorpowerwashing.com
House Cleaning
Fleet Washing