I need to seal a small area on exterior of my
house - it's painted block. Home Depot says
I have to repaint - that there's nothing to
waterproof it once it's painted. Can that
be true?

I'd appreciate any ideas/suggestions.

False. You may apply a "solvent-type acrylic coating" over paint which is adhering well. Such a product is Modac, manufactured by American Coatings Co. in Pennsylvania. I believe they make a clear coating, and several colors. I believe there are other such products on the market also.

This coating will repel water even from a fire hose, but will still allow moisture to pass out from within the wall. It lasts 15-20 years without recoating. It's expensive and it goes on heavy, something like 30 square feet per gallon, compared to 300 square feet for conventional paint. It is roller-applied.

Thanks, Richard. I really do appreciate it.
I just couldn't believe that there was nothing
out there that would work and you proved it.
Thanks, again.
Betty

Hi, What if the exterior is painted already and not so well? Our house is 30 years old and the paint is white and ... powdery. We've toyed with stripping the entire outside and residing.

Any suggestions?

Try Google for "Modac" and hopefully you'll find their instructions. I KNOW it can go on over paint which is adhering well...I don't know about paint which is chalking...but I'm sure their literature will cover that issue. If not, call them. I believe the company who makes Modac is American Coatings Comapny, located somewhere in Pennsylvania.