An old coating
Can a new coating go over an old one?
Usually the old coating comes off first, because a new coating is only as attached to the floor as whatever it was put on top of.
The short version
- A new coating is only as attached as the layer under it, and an old film's bond cannot be checked from the top.
- New material often cures tighter than what is under it and pulls the old film off the slab.
- The usual answer is that the old coating gets ground off and the new system goes onto bare concrete.
- Removal is slower than grinding a clean slab, and it shows up as its own line on a quote.
- Tell whoever quotes it what is on the floor now, and send a photograph of an area that has let go.
The problem with what is already there
Nobody knows what the old material is. A previous owner rolled something on, or a kit went down before you bought the house, or the floor was done years ago by somebody who is not around to ask. Which product it was, what the concrete was like underneath it, and how well it was stuck the day it went on are all unknown, and all three decide whether anything can be built on top of it.
That matters because the old bond becomes the new floor's bond. A coating put over another coating is holding onto that one, and that one is holding onto the concrete. The finished floor is only as good as the weakest joint in that stack, and the weakest joint is the one nobody can see or vouch for.
What usually happens next
New material is often stronger and tighter than what is under it. As it cures it pulls, and a modern coating pulling on an old film that was never well attached tends to take that film off the concrete and bring the new floor with it. What comes up comes up in sheets, and the underside of the sheet is smooth concrete.
Which is why the usual answer is that the old material comes off. The floor gets ground until what is left is concrete, the profile gets opened in the same pass, and the new system goes onto a slab rather than onto somebody else's work. Where the old coating is thin and mostly failed already, a lot of it lifts with the first pass.
What that does to the quote
Removal is slower than grinding a bare floor and it is harder on the machine. A coating that is still well stuck fights back, and a soft or gummy one loads up the diamonds and has to be cleared off them. It takes more time, more consumables and sometimes different tooling, and all of that shows up as a line on the quote.
So it is worth saying on the phone that there is something on the floor already, and worth sending a photograph of an area where it has let go. Whoever comes out then arrives knowing what they are looking at, and the number you get back already has the removal in it.