How to clean up paint on cement

Painting is easy - preparing for a new coat of paint so it will last is the hard work.

You should scrape off all loose or peeling paint - otherwise the new paint will simply peel right off there. Only after that should you move to spackle and sanding to prepare the base for new paint. On wood items "putty" is preferred over "spackle" which is more of a drywall/plaster repair item, though it's somewhat irrelevant when it's being painted over.

The window looks like you should investigate for and fix a leak. If it gets that wet, new paint won't stick either.

The dark spot in the second picture is in the same place on all the pictures so it's a camera defect, I guess - at first it looked like mold.

The blisters in the third - more leakage? Or pitch bubbling out of the wood?

I would try a Muriatic acid and water solution. I know the Acid will etch the Block just not sure how well it will remove the sani-tred. I would start with a 10% acid solution and work up from there. Usually I do this to floors prior to painting. If I need to make the surface rough for better bond for tile it works also. I have used mixtures up to 30% depending on how aggressive I want to etch the cement. You will need fresh air in a basement as the fumes can get quite strong. Remember to always add acid to water and use rubber gloves to protect your hands. All pool supply stores will carry Muriatic acid and most home improvement carry it. It is not that expensive for a gallon to give it a try.