Skim coat on cinder blocks

Go to a paint store and tell them what you want to do. They will sell you a blocker/primer.

You may compare the cost of the blocker & paint versus a skim coat of plaster. The fiber-reinforced plaster is not inexpensive and I wouldn't paint it afterwards. You may not feel comfortable troweling it on either.

I just put this on another thread but I'll repeat it. There is a new product out called Fibra Fuse it comes in 36" width rolls and it is really good for skimming. Having said that I have no idea about concrete. I would go to their website and try to find out or call their 800#

If the blocks are exposed on the outside, they'll absorb a certain amount of moisture during a rain event. Any plaster type of coating or drywall compound will fail from the moisture absorbed. You could coat it with something like Thoroseal

http://www.thoroproducts.com/products_waterproofing.htm

or parge it with portland cement. Cinder blocks are even worse than concrete block since they are more porous.