I water proofed my basement wall about 5 years ago. I then painted over the waterproof paint with a high gloss white pint to make the room look bright. The foundation is cinder block about 80 years old. It is the old diamond shape cinder block. One section developed yellow rings and the water proof paint began to separate from the cinder block. It is sporadic over a 10 foot section. Last summer I graded the outside of this area to make sure the water would run off away from the house. I installed a plastic window cover outside so the water did not run into the window sill. The basement is not a damp basement. I run a dehumidifier just to play it safe. I think this resolved the problem

My question: I want to clean/scrape the area of the wall that has the yellow ring marks and some powder dust (I suspect this is the mold) and re-waterproof and paint. How should I proceed and what materials should I use?

A stiff wire brush should suffice to remove the loose paint and then apply more waterproofing (like Thoroseal) AS LONG AS you are confident that the original waterproof "paint" that you used was truly a waterproofing compound (Such as Thoroseal).

If your original waterproofing was not something that penetrates the block, then you should remove it too.

Waterproofing like Thoroseal penetrates the block. Putting it over a "paint" will not do anything since it will not be able to penetrate the paint layer.

PS. If the stiff wire brush does not remove the paint layer, try an attachment for your drill that has long wires on it. Something like this: http://www.pjtool.com/ProductImages/powertoolsandacces.-drillacces/1867-C.jpg

Hello -

I seem to have the same problem and was told that some mold has grown. The contractor suggested I remove the crumbling paint with a wire brush, spray the wall with a mold killer or bleach and water combo, and then repaint.
This is a question based on the original question. Is there a way to tell if the current paint is a waterproof sealant? I believe it is but the previous owners painted the wall, not me.
Also, will there be noticeable lines where the original sealant and the new sealant meet?
Thanks,
T.