Pistons have coolant ***help***

I always leave the old coolant and oil in the engine when I do a head gasket. Changing fluids after the job is done carries all the bits of gasket material and gunk out with them.

I always mop up as much coolant as I can out of the cylinders with rags, roll the motor over to bring all the pistons to TDC to dump as much the rest over the side, then spray the cylinder walls liberally with WD-40 (WD = water displacement, what this stuff is actually for) and then wipe the cylinder walls with a clean rag.

Never had a problem with this method.