Many boat owners feel that a compass with one or two small bubbles is somehow defective. This could not be further from the truth. Sometimes, bubbles in a compass are due to changes in temperature. There is probably a problem though, if the compass has recently been hit or cracked, resulting in a broken dome or damaged seals. In this case, sending the compass out for professional repair is the only alternative. If this is not the case and the bubbles are annoying to look at, they can be removed.
Remove the compass from the boat, following the instructions in the boat manufacturer's service manual and using whatever tools are necessary from a toolkit.
Turn the compass on its side, ensuring the filling screw is facing upwards. Carefully remove the screw, following the compass manufacturer's instructions and using whatever tools are necessary from a toolkit.
Fill an epoxy syringe with compass oil and insert the syringe into the filling screw hole. Squirt the compass oil into the compass until it is full. Rock the compass back and forth and from side to side to allow the air to escape from the compass. Add more compass oil, as required.
Replace the filling screw and set the compass up right. Inspect the compass to ensure that all the air bubbles are out of the compass. Repeat the process if further air bubbles are discovered, until all the air bubbles are completely out of the compass.
Compasses built before the 1950s can be filled with alcohol. Most modern compasses use a combination of diluted mineral oil.