Sounds like incompatibility between the layers of paint, which can happen when you put latex on top of oil paint for example. In one of our past houses you could peel the latex "enamel" crap off the woodwork in large sheets--which is what I did-- peeled it all off, lightly sanded and repainted with an oil enamel.

Unfortunately your answer is probably to not put anything on the sills or disturb them in any other way or to strip it off and start over again.

Hi rwiegand, thanks for the reply. I'm pretty sure though that latex over oil paint isn't the problem. Our apartment is new construction and I'm 99% sure the contractors used latex paint only. Our last apartment had the same problem and there, the contractors used latex primer & paint on sanded down wood sills.

I was told that you want to use oil based paint on anything that you will be stacking stuff; bookshelves, cabinets, etc. otherwise this will happen. This is only what I have been told and as long as I have stuck with this, I haven't had a problem.

If you're sure it's latex on latex, are you also sure that it's flat on flat, or satin on satin, or semigloss on semigloss? Generally you can make it glossier and get away with it, but a more matte/flat will never adhere to a glossier undercoat. That will cause the same problem. I agree with using oil enamels on windowsills, shelves, etc, you can USE latex on windowsills/shelves, but it wont have the durability you need if you plan to put items on them. I think you're going to have to strip it, sand it, prime it with an oil primer, then paint it with an oil enamel, and give it a week to cure completely before putting heavy items on it. If you want to be extra safe, dust the little felt pads you put on the pots with a dusting of cornstarch, that will help reduce any tackiness between the surfaces.

Ricky & Phaewryn - thanks for the tip on oil-based paint. I had no idea it was better for window sills. The painters I hired to paint my last place used the same paint for the sills as they did for the rest of the trim, latex semi-gloss. They never mentioned this issue so I assumed latex was fine.

Another question - sometimes in extreme cold, we get condensation on the window frames that pool on the sills, causing water damage to them. I'm thinking the oil-based paint will help repel the water and avoid this issue. Is that correct?

What nobody wants to admit is that latex paint is crap.

What you can do is apply clear acrylic varnish (like Minwax brand) over the cured latex paint.

Unfortunately no paint on wood will long resist standing water, so changing won't help your window condensation problem. For that you need either to decrease your indoor humidity or improve the insulation value of your windows by, for example, adding storm windows.

I can't agree that all latex paint is crap, but for woodwork oil is a vastly superior choice because it's so much tougher.

Thanks for all the suggestions. I'm tempted by the seeming ease of using the acrylic varnish, but I think I will go the long, harder way of strip/sand/repainting the sills with oil-based paint. Any suggestions on going about this? I will do some research, but I always appreciate advice from people who know more than I do.

I am not seeing what brand of paint was used, but I have been doing this a very long time and any quality latex( or water borne) will not do this. At least it never has for me and my personal window sills are almost 2 feet deep and I put potted plants on all of them. I happen to use Ben Moore's Satin Impervo water borne the last time

I agree it depends on the brand used and what you are painting over (how well the prep was done on previous painted surfaces). If you are using some mid-grade paint, it will not be as durable and the cured paint film will not be as hard as high quality paints. As for the condensation on the glass leaking down on the wood surfaces, no paint will hold up forever when you allow water to sit there. You may want to mess around with adjusting the humidity level in your home to minimize that condensation that forms on the glass.

I think there is a zinssner primer out there that is supposed to stick to any type of paint and then can be coated with any type. The other guys can probably tell you if this works well or not, but maybe you can just scrape all the loose paint off and then put this on before the oil paint and maybe save you a little time. Usually if its a shortcut you pay for it in the end, but if anyone has used it maybe it really works.

the windows that have condensation issues, are they single pane? do you have storm windows in?

I know that this is old, but a couple of addenda for those running across this in a Google search: you can actually coat over glossy paint with flat, but you first have to sand the glossy paint until it's dull (thus creating a porous surface that the new coat of paint can stick to), or use a primer that will stick to glossy paint. The primer rickyk22 is referencing is probably Bullseye 1-2-3, which does indeed do a good job of sticking to gloss (I've tested this up to semigloss).

Latex paint will hold up on a windowsill, but a high quality paint is recommended for anything that's going to take regular wear - aside from the issue of pulling paint off, you may notice lower quality paints flaking or cracking due to humidity, UV, and temperature changes from the window. Glossy finishes will be more resilient to this than flatter ones.