I've bought some bearings, and heard its a good idea to pack them with marine waterproof grease before using them.
On the tin of the grease it says not to mix with other greases, so what is the best way to remove the preinstalled grease in the bearings? isopropanol??? white spirits??

James

White spirit will work a treat.

putting new grease in should force the other stuff out IME. shve it in from one side, pushes the other stuff through the other stuff. and im pretty sure a bit of old grease mixing with the new stuff wont do any harm.

Why bother (unless you are kaesee). All you will achieve is;

*Too much grease, bearings can't move smoothly, accelerated wear.
*Nick/bend/damage the seals, crap gets in, grease gets out, accelerated wear.

Just buy SKF or comparable bearings, they know more about what they are doing than 99.99% of foum users (including probably, me).

Edit: In answer to your question, nothing shifts it quicker than comprssed air (photography shops etc sell it in cans which last quite a while).

rub them with a cat

go with the fur, not against

Jamiep beat me to it 🙁

Get some brake cleaner, puncture the base of the can with a can opener thing, pour out enough to cover bearings, give a shoogle about, and get the grease off. Pour the rest onto a cloth and either mug people or put over face and recreate anesthetics of days gone by (best to have willing partner to remove cloth before dying)