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If the residue on the block, which are actually lime deposits, is really bad, then scrubbing it off will be necessary. You can buy a good concrete block cleaner form your local home improvement center. BEHR and ZEP are two brands that come to mind. You can use a hard bristled brush or masonry brush to scrub with.
We talked about Drylok in the last question but there is another really great product line out there called Xypex. This is a remarkable product that actually is made to not just penetrate into the block but do more. Xypex creates a crystalline structure deep within the pores and capillary tracts of the concrete mass to prevent the penetration of water and aggressive chemicals. In contrast, barrier-type products function only at the surface of the concrete.
To find out more about it click on the following link:
You can also read more details about in one of the previous questions on Ask Rick by clicking on the following link.
http://www.showroom411.com/AskRick/Question/water-basement/25
This product can be applied to a damp wall and part of the process is to keep it moist for a few days so that the crystalline action takes place. Once the process is completed it is like it becomes a part of the block wall, keeping water out.
When I checked last you would need to order this product online from their website. You could also ask them if there are any dealers in your area.
I hope this helps. Rick
Rick Maselli is Founder and Editor of Showroom411.com and Ask Rick.