As others have said, trend is king. I find mine pretty accurate, to about 0.004-6 ish. Easily good enough to follow the trend and get a good idea if it's hitting around the FG that I'm aiming for. I always take manual readings for start/end and to determine my efficiency etc. as I brew.
Small bubbles on the side will make very very little difference, and within the expected error margin of the device. I wouldn't worry about any coatings.