My Canon CP800 was printing OK yesterday but when I tried it today the first photo had a white line along it. I fitted a new cartridge and it did the same. I printed a test page and there is a line on the photo paper as if something is cutting into the paper. Is there a way of cleaning this printer.
     Try to clean the print head of you printer, maybe there is something mess to your printer head or try to clear the clog. It is recommended also to submerge the print head in alcohol or pressurize the head with alcohol or with water, or you can use also other cleaning fluid with using a syringe. After cleaning, it is very important to make sure that the print head before re-installing it again to the printer. It can damage computer if you put it wet because there is electricity in printer.  You can try to look at this site for pictures on how to clean print head. http://www.pbase.com/muzzy996/print_head_cleaning
Hope this helps.
If you are trying to print a photo with a Canon printer and you got white line on the print, try changing to a much thinner paper. If this doesn’t work and fine lines are visible in images printed with Canon SELPHY printer, the problem could be because of dust inside the printer. Try removing any dust from the printer to remove the fine lines in the print.
Here’s how to clean the back side of a Canon SELPHY CP800 printer. If there is dust on the battery cover or air vents, detach the ink cassette then remove the dust. If dust comes in through the air vents, it will stick to the ink sheet. As a result, you won’t be able to make clean prints.
If dust gets into the ink sheet, you can use a blower brush or something similar to remove the dust without touching the ink sheet with your hands. To detach the ink cassette, move the battery cover lock upward then pull the ink cassette.
To remove or detach the battery cover, move and hold the battery cover lock downward then move the battery cover outward. After this, remove dust from the air vents.
