Non-serviceable bearings
Many bearings have metallic shields which are not made to be removed. Therefore the bearing cannot be serviced. An example is the SKF standard industrial 608-2Z.

But you still can maintain them!

Dirt on the outside face of the bearing needs to be cleaned off. When it's cold outside moisture is condensing on the bearings. The outside needs to be dried.

Just dismantle the wheels from the chassis. Use a dry piece of textile or a smooth dry toothbrush to remove the dust or moisture and foreign particles that stick, in particular where the grease oil leaks out from the outer shield of the bearing. Do this from the inner diameter to the outer diameter of the bearing. (see photo)Afterwards just put some grease on the metallic parts and spread a thin film for protection from rust. You cannot do more than that but it is enough to avoid rust here!

Serviceable greased bearings
SKF SPEEDtrainer and SKF GOLDracer bearings are semi-serviceable. This means that only one of the two protections can be removed easily to clean and re-lubricate the bearing. Do not remove the seal on the other side!

For SKF SPEEDtrainer bearings, SKF has developed easy removable black non-contacting seals. For SKF GOLDracer bearing sets, SKF also has developed an easy removable helicoidal shield which is patented under the name "Quickshield". For both solutions seals are easily removed with a pin.

Warning! Before removing the shield or seal and to avoid foreign particles entering the bearing, please clean the outside of the bearing as stated above.

Cleaning the inner parts of the bearings:
SKF generally does not recommend the cleaning of the inner parts of the bearings. Nevertheless, if you want to, please take care of the cleanliness of the cleaning product and of the cleanliness of the place and equipment used to do that. The cleaning product that is used in SKF factories is filtered to avoid particles bigger than 0,4 microns! So you take risks cleaning bearings, definitely if the with products that are not clean enough to do so.
For information find below cleaning solutions ranked from best to worst:

Ultra sound machine plus continuous filtered cleaning product
Ultra sound machine plus cleaning product
Cleaning product in pressured bottle into bearing (example WD 40 sprays)
Bearing shank in cleaning product that is changed several times
Flow of cleaning product into bearing
Bearing shank in gasoline
Bearing shank in cleaning product that is used
No cleaning.
The most common way of cleaning is as per ranking number 4:
For SKF serviceable bearings we recommend to introduce the cleaning product through the gap between the metallic shield and the inner ring of the bearing. So, the dirt can go out of the bearing through the open side. Of course, do this after having cleaned the outer surface of the bearing. After cleaning several times (each time with new cleaning product) dry the bearing in a place protected from dust, closed side on top (dust will have a lot of fun coming into your bearing through open side while drying). When your bearing is dry put new lubricant in it. For standard use SKF recommends SKF grease LGLT2 in quantities to your rolling willingness. The more you add grease, the less the bearing will roll, because grease always adds friction. Press in the quickshield or the non-contacting black seal to close the bearing.

Before mounting:
Did you protect the outer surface of the bearing?
Did you clean the hub of your wheel?

Oiled serviceable bearings
SKF SPEEDracer mini bearings and SKF SPEEDskater bearings made by SKF are designed for racing. Racers like to use oiled-bearings because they roll better than greased ones. Nevertheless they also need regular oil re-lubrication. For this reason our racing bearings have an open and a closed side.

Want to re-lubricate regularly?
Just dismount the bearing from the wheel and put 1 or 2 drops of SKF Speed Master Oil in the bearing. Mount them after having cleaned the hub of the wheel.
Want to clean inner parts of the bearing?