Paint stripper slows down in cold weather. I had to say that in case it's cold where you are. Optimal surface temp is between 75 and 80f.
If it's a paint that's really impermeable to methylene chloride at the proper temp., then you probably have a casein or other protein-based paint. Try some ammonia, not the kind w/soap or detergent. That should break down the proteins. Strong ammonia also discolors many woods, so you really don't want it to sit longer than necessary. And you have to wear the proper skin/eye/lung protection. IMO this is a warm weather, windows-open kind of job.
Casey

I stripped really old paint from a staircase built in 1881. Same scenario, tons of old old paint that was not easy to remove. I had the best luck with ZAR semi paste paint and varnish remover using a putty knife, steel wool, a typewriter brush, a ceramic cleaning tool and twine. (the last 3 to get into crevices)

I found that if I put the stripper on and let it sit for 10-15 min until the old paint actually bubbled I could get the goop off.

It took me almost 6 months to do the entire staircase working on it in the evenings after work.

Hardest part for me were the intricate spindles though. the stringers and risers were cake after those babies.

I've never worked on anything really old, but I've had good luck with Citristrip. Put it on thick (think frosting more than paint), and cover with plastic, to keep air out. It keeps working as long as it's wet. Let it sit for 24 hours, then scrape and rinse with water.

Good luck - I'd love to see it when you're finished!

Milk paint is impervious to strippers. There is a Behlen Masters product, - P.D.E. paint remover that will remove it. Needless to say you will not find it in every paint department simply because there is so little demand for removing milk paint.
Heat works when you get the right combination but be forwarned,heat also desolves hide glue so the stairs may fall apart.

Hi - Thanks to all of you for your suggestions. I got caught up in a (non-DYI) project and didn't have a chance to reply sooner.

Sombreuil_mongrel - Good info. I didn't know that the optimal surface temp was that high. This is an indoor project, and the air temp was probably about 68-70 degrees, not so low that it wouldn't have worked, even it needed more time. However, maybe my stripper wasn't as fresh as it should have been, I was using stripper from a can I opened about 2 months ago - would that make a difference? I was using a paste stripper which may not have as long a shelf life as a liquid one.

Also, I forgot about the possibility of milk paint. I didn't have any ammonia in the house, so I picked some up to test. If I wipe the paint with an ammonia-soaked rag, the color comes off on the rag. (Unlike the other layers, which are white, the problem paint is dark and appears to be a mixture of green, red, and brown.) I wouldn't say that the paint is really dissolving, though. If it is milk paint, how do you use the ammonia? Just keep wetting it and scraping? I'm not worried about discoloring the wood, since I'll be repainting it. (The paint was in terrible shape - there was no way I would be able to get an acceptable paint job without removing the old paint.)

weedyacres - I didn't want to try a heat gun for I couple of reasons. I knew someone (a smart, generally responsible person) who set fire to his home using a heat gun to remove paint. If he could do that, just about anyone could. I don't really like working with chemical strippers, but I'm too nervous to use a heat gun, especially since I'm working indoors. Aside from that, I'm working on a stairway with no outlets nearby, so I'd have to use a long extension cord, which would be a tripping hazard. I do own a heat gun, though, and I'll try it the next time I have a project where I can work outdoors, away from the house.

petchie - Six months? You deserve some sort of award. Must look great. (Maybe that's the reward.) Fortunately, my spindles were never painted, which is why I thought THIS PROJECT would be the proverbial piece of cake - flat surfaces, small work areas, no intricate detail. I thought I could manage 2 or 3 risers an evening and was in for a shock. This paint is unlike anything I've ever worked with.

Btw, it occurred to me that the abrasive kite string (manjha) that they use in places like India and Afghanistan for competitive kite-flying might be a great tool for getting into grooves. It's covered with crushed glass. (The idea is to cut down your competitor's kite by sawing through its line,)

annkh - I like Citristrip too, when it works. It's so much easier to use. It was the first thing I tried on my stairs, but it only worked on the top layers. I tried re-applying, but it didn't seem to have any effect on this particular paint. It was terrific on some of the other woodwork I stripped that had normal paints, although I had to do at least two applications, once for the latex paint on top, which separates from the other layers, and once for the older paints underneath.

klem1 - Thanks for the tip. I didn't know about the Behlens product. I'll look into it. Do you know of any brick-and-mortar stores that sell it, or do you have to buy it on the web?

Thanks again everyone for all the help!

Here is a link that might be useful: Manja or manjha - abrasive kite string

Hi,
I used steel wool to scrub it; keep dipping the steel wool in ammonia. Such fun.
You don't have to neutralize, as ammonia evaporates thoroughly, although faster the warmer it is.

Casey

Thanks, I'll try that.

And speaking of neutralizing, when you use methylene chloride stripper, do you need to use a commercial afterwash from the same company that made the stripper, or is there something else you can use? The reason I'm asking is that the one I bought smells an awful lot like acetone, which would have been much cheaper.

If all else fails, get a can of Zip Strip. There's almost nothing it can't remove,and does so very quickly. Be sure to use ventilation with its use.

I'm stripping my stairs also 1890's house. i've been doing it on weekends, it's a winter project that I started at the end of october. i start with the heat gun, then the stripper and scrapper (i have goop too, i think it must be some kind of base coat, it's a light beige/gray, they used before doing a varnish faux finish). i finish off with more striper and steel wool.
I wear a mask and leave the door open a crack.
It's a job and a half....good luck

I'm stripping my stairs also 1890's house. i've been doing it on weekends, it's a winter project that I started at the end of october. i start with the heat gun, then the stripper and scrapper (i have goop too, i think it must be some kind of base coat, it's a light beige/gray, they used before doing a varnish faux finish). i finish off with more striper and steel wool.
I wear a mask and leave the door open a crack.
It's a job and a half....good luck

I tried everything to strip the paint from woodwork in out 1865 home and finally, after many months, gave up. I sanded it all to a smooth finish, applied a base coat, and grained it to look like walnut (took classes at a local arts center on faux finishing). Best decision that I could have made.