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This is a pillar supporting an enclosed porch roof on a 1930s Craftsman in Northeast US. The porch is usually unheated during winter but it is entirely enclosed and not subject to rain/snow.

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As you can see this has been painted at least a couple times but the concrete is loose and crumbly. I would like to remove the loose material, and depending how much is left either seal and paint or else skim some new (epoxy based?) material over to replace what came off and bring it back to a plane and then paint.

Assume I am not skimming on new material: is there a quality product which can “seal” (strengthen?) the concrete so it will not continue to crumble away under the paint I will apply? I assume I would apply an oil based exterior paint - but is there any reason this would be counterindicated?

Assuming I do find I should add new material on top of what has integrity below the loose/crumble - what is a quality material I can skim on and will “bond” strongly? I believe an epoxy product is typical for bonding to concrete but do not know how to select from these or if there are any tricks to do it right. I will paint over this new material and don’t want blistering or chunks falling off in a few years.

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This will be a never-ending battle, also called "maintenance." All you have to do is prepare some Type-N mortar (1:1:6 lime:cement:sand) and apply it. Repeat every 10 to 30 years, depending on the severity of the erosion.

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Crumbly concrete bricks from the 1930s? I believe that's what actual cinder block looks like. I've never seen the stuff, but that's my bet. If you start removing the crumbly stuff, there might be nothing left by the time you're done. There's "block filler" paint for priming that surface for paint and maybe inhibiting the crumbling. Unfortunately there's no wood hardener analogue for concrete or coal waste or whatever that is.

A stucco coating might be best. That porous substrate will definitely grab onto the mortar. There are surface coatings and mortar additives to improve adhesion if you find tougher material beneath the crumbling stuff and you want to minimize risk. Sika also has a product that sounds tuned for this sort of thing, Sika 4012 Monotop, where I might try to get them on the hook for helping resolve the problem if it shows up again.

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