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Prep and painting over previously painted concrete floors

I'm getting conflicting information surrounding prepping and painting over a previously painted concrete floor. Here's the situation: I’ve got a concrete basement floor that's been previously painted multiple times with what appear to be different types of paint. Some areas were bare concrete, some areas have what appears to be two different coats of 50's or 60’s vintage epoxy paint that are pretty well adhered. The previous HO decided to spray a coat of cheap latex paint over the whole mess. Even something as simple as a combination of spilled water and boot scuffs is causing the last layer of latex to come off. I've started to use a water based (non methylene chloride) based concrete and masonry paint remover to remove the layers. Its working slowly but it’s a ton of work and is giving me very uneven results. What’s the best method to prepare the floor for an eventual coat (or coats) of epoxy or other concrete paint? Are there specific cleaners and or primers to use? Does this whole mess need to be stripped back to bear concrete? I’ve heard that sanding with a course grit will suffice. Unfortunately pressure washing the area isn’t the option because a good chunk of it is in a finished space.

Re: Prep and painting over previously painted concrete floors

Swiss carpenter,

For sure the last coat of latex which is not bonded must come off. Any new coating is only as well adhered as the coat it is sitting on!

Your choices for removal are either chemicals or grinding. I am not sure that grinding might be the lesser of the evils. Rental centers have machines which look like upright buffers, but have diamond stones to grind the surface. The pros who do garage floors use these to remove old finishes and roughen up the surface.

In a basement situation, I would consider a one part epoxy for the finish coat. If you are down to bare concrete, there are clear primer/bonding agents to help assure good adhesion.

Re: Prep and painting over previously painted concrete floors

Thanks for the info. I looked into scarification but unfortunately the rental yards in the area only stock machines that are gas powered and don't have a vacuum. Also the dust generated would be prohibitive in my basement, so sadly I cant go that route. The good news is the chemical stripper I'm using is leaving me with a combination of very well adhered epoxy paint (most likely the first layer from the 50's) and a good amount of bare concrete , so I feel fairly confident that my new paint will hold. Do you know of a bonding primer that will hold to both the concrete and old epoxy? Or heck, do i even need to use primer with two coats of epoxy paint?

Re: Prep and painting over previously painted concrete floors

Swiss Carpenter,

The hard 1950's finish you are getting down to is probably not an epoxy, but a hard alkyd oil floor enamel. In any event, a 2-part epoxy should go right over that which you can't get up. The 2 part epoxy floor paint probably does not call for a primer.

Behr does have a primer for use on bare concrete, but is not to be used over old paint. It is recommended for use under the Behr One Part Epoxy, but not under the two part epoxy.

The success of any floor paint on concrete grade depends on if there is vapor pressure present. You can test for that by taping a piece of heavy plastic garbage bag to the floor for 24 hours. If there is condensation on the plastic when you pull it, there is moisture present. You can still paint it, but you will in all probability have some paint failure in the future.

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