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Prep­ping painted con­crete.

Q:

My out­door porch has a con­crete floor and steps. It was painted, prob­a­bly 50 years ago, with brick-red paint, and re­painted more than once with la­tex paint, which did not ad­here well. The red paint has worn in places, but ap­pears ex­tremely durable. The Rust-Oleum con­crete paint strip­per and 3000 psi power washer I used to re­move the la­tex did not faze the orig­i­nal paint. I would rather re­paint with a more sub­dued color. How can I re­move the old paint or get la­tex to ad­here? — Neil Wolf, via email

A:

The an­swer is not as ob­vi­ous as I thought. Con­crete it­self presents ad­he­sion prob­lems, be­cause it's por­ous and al­ka­line, and walk­ing sur­faces are es­pe­cially prob­lem­atic. New, un­fin­ished con­crete is easy enough to deal with given mod­ern prod­ucts. Pre­vi­ously fin­ished sur­faces are harder.

The red you see may be a stain ap­plied when the con­crete was wet, mak­ing it in­te­gral, or it might be an oil-based coat­ing (in a for­mu­la­tion no longer made), or it may be an early, post­war epoxy. If you can get the floor and steps ab­so­lutely clean, with no em­bed­ded dirt and no grease, and a sur­face with no peel­ing or other paint fail­ure, you should be able to re­paint suc­cess­fully. First, all sub­se­quent la­tex paints must be com­pletely re­moved.

Looks like you al­ready did a good job, us­ing the right paint strip­per and power wash­ing. If any stains re­main, clean with TSP or an oil-stain re­mover from a com­pany like SEAL-KRETE; rinse, and al­low to dry. Be sure all mildew and mold are killed and stains re­moved. If the sur­face is very smooth you may need to etch it with HCl acid; you can buy acidic etch­ing liq­uids from Rust-Oleum and ce­ment-prod­ucts com­pa­nies. I think it’s un­likely you will need to do this on a weath­ered ex­te­rior sur­face. (Please use full safety equip­ment when us­ing TSP and acid.)

All patch­ing should be done and have cured. You do not need a sealer. The floor must be thor­oughly dry. Since you will be prim­ing and then ap­ply­ing two or three thin coats of masonry paint, with 24-hour dry­ing pe­ri­ods, you’ll need up to four days with no pre­cip­i­ta­tion and, ide­ally, low to av­er­age hu­mid­ity. Ap­ply one coat of a bonding primer to as­sure com­pat­i­bil­ity and ad­her­ence with the old fin­ish. Whether acrylic or oil-based, this should be a for­mula specif­i­cally for masonry; it may be la­beled “elas­tomeric." Use a roller; do not spray.

I was sur­prised to find that epoxy coat­ings, the most durable for con­crete walk­ing sur­faces, are not rec­om­mended for pre­vi­ously painted con­crete, es­pe­cially if the paint was oil-based. So that means you should use a masonry fin­ish paint spec­i­fied for floor sur­faces, one com­pat­i­ble with your primer. Both acrylic la­tex and oil for­mu­las are avail­able. (Sher­win-Wil­liams Porch & Floor Enamel, for ex­am­ple, is la­tex and can be used on con­crete.) Use a spray gun or brush and roller. Ap­ply at least two thin coats, 24 hours apart. Note that op­tional paint ad­di­tives can lend tex­ture for “grip” (safety). Please con­sult with a knowl­edge­able per­son at the paint or masonry store when you buy prod­ucts.

—Pa­tri­cia Poore

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