should paint come off with a razor blade?

We want to repaint a plastered room. It has 2 (3?) previous coats of uncertain age. Some paint is coming off, at least partly due to water from a long-ago roof repair. I scraped it pretty aggressively. In some areas, I got down to the plaster; in others, the paint seems intact.
Where I did scrape paint off, I find that I can attack the exposed edge with a razor blade, and pop the paint off in small patches. Mostly, this is just the top coat, though sometimes, it gets down to bare plaster. It's not clear how far I could go with this - at the least, I can get off a LOT more than the scraper did.
The question is, whether this is 'normal'? It doesn't seem like well-bonded paint, but maybe it's OK? It would be a PITA to do the razor thing on the whole room, but there's no point in new paint that's going to come off, either.
Thanks
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On Friday, January 22, 2016 at 2:35:45 PM UTC-5, snipped-for-privacy@gmail.com wrote:

It's not normal for paint to come off with a razor. But on the other hand, if the areas where it's coming off on it's own are limited to areas that had water damage, then what's there is probably bonded well enough that it doesn't need to be removed, which is a major PIA. If the areas where it's coming off are limited, I'd just deal with those and then put a primer over the whole thing.
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On 01/22/2016 2:13 PM, trader_4 wrote: ...

I'd think it actually quite normal... :) Attack a hardened film layer at it's edge with a wedge and it's pretty much simple physics to raise that edge.
That said, if it takes that level of effort to remove, it's likely "good enough".
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I would try to avoid too much sanding if there is a chance that the plaster has asbestos in it - ie 1960's. Getting it properly tested for asbestos might be costly - and it's one of those things you might rather NOT know .. John T
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On 1/22/2016 8:10 PM, snipped-for-privacy@ccanoemail.com wrote:

You can remove paint easily with no tools. One night for dinner we had a lot of onions and mushrooms. Next morning I was able to peel the paint off the bathroom walls and ceiling. Peeled off in sheets.
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By the time you sand and scrape and pay for asbestos and lead paint tests and all of that, why not just sheetrock the room! -OR- Cover it with paneling.
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On Fri, 22 Jan 2016 14:35:42 -0500, snipped-for-privacy@gmail.com wrote:

Old hardened paint will do this. Just get the loose paint off. Use drywall compound to smooth the wall, then paint.
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On Friday, January 22, 2016 at 2:35:45 PM UTC-5, snipped-for-privacy@gmail.com wrote:

we had and fixed that problem here, and its been fine for about 20 years.
remove everything loose, really work at it. then wash with tsp and rinse well. let dry for a couple days. then paint with a bonding agent so the next layer sticks well..
then skim coat with drywall mud. let dry well.
then paint.......give it 2 coats
like i said near 20 years ago and zero problems.
avoid sanding because of lead based paint, and possible asbestos
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On Fri, 22 Jan 2016 14:35:42 -0500, snipped-for-privacy@gmail.com wrote:

Most likely someone painted latex over oil based without an intermediate primer. Typical DIY mistake. Sand out to suit yourself, spackle and repair as needed, prime with an acrylic primer (not a primer + paint combo), then top coat.
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