Rats can develop skin problems for a variety of reasons ranging from dietary deficiencies or parasites.
Sometimes a rat can be allergic to a specific food like peanuts or processed pet foods. To eliminate these skin issues you need to get to the source as fast as possible. In the allergy scenario, all you need to do is remove the offending food from their diet and the problem should subside on it’s own.
However, if it’s a deficiency, try and determine what it needs and give it to them. This could be a lack of vegetables, meat or grains. If there are scabs already on their skin, then make sure you treat them with some antiseptic ointment and clip their nails to prevent further damage. If mites are determined to be the culprit, then get hold of some Ivermectin. This is a liquid formula that can be absorbed into the skin. There is also an inject-able version but many rats have adverse reactions to this. This needs to be given to them every two weeks until the problem disappears.