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We are moving to an apartment built in the 1920:s which have a fillet between the walls and the ceiling. We want to paint the walls, and to get a nice finish between the ceiling and the walls, we want to paint a distinct line around 10 cm down from the ceiling. See attached image.
What would be a good way to make a straight line, when the walls and/or ceiling is not perfectly straight? I'm thinking perhaps you could make a jig which fits in the fillet?
I've had mixed results with tape. Paint will wick/bleed under loosely bonded tape, especially if your surface is rough and well-bonded tape can damage the surface especially if you leave it on too long. A friend of mine likes Frog Tape, but I've never tried it. I just free-hand it and switch to decaf coffee for a day! I got good pretty quickly. and it's not like tiling or something where you can't fix your mistakes. If you don't mind doing touch-up the day after, it's usually faster than taping.
As far as positioning a line that goes with the flow of the ceiling, I would make a jig by screwing a few blocks of wood together that holds a pencil. Use it to mark the wall and you'll have a visual guide to follow when you freehand. The yellow arrow in the sketch below represents the pencil and I would use a big enough block that averages out maybe a square foot or so of ceiling. if you see sections of the line you don't like (because the jig hit a bump or whatever), just erase the offending section and use a long straight-edge like a 4-ft level or strip of trim to redraw the line.
Edit: If your ceiling is very uneven, attach three (not four) "feet" to the top of your jig. Doesn't really matter where, just position them away from each other in a triangle. This will prevent the jig from rocking on the high spots.
Use masking tape to isolate the area of the line to be painted. You don't say how wide the line needs to be.
Here is a trick for making sure the two pieces are perfectly parallel. What you do is get two kinds of masking, one wide and one narrow. On a long flat surface you lay out your wide tape and secure it to the surface sticky side up with tape or tacks. Now dust the tape lightly with flour. This will reduce its stickiness. Then, put two bands of narrow tape, sticky side up, on the wide tape. Since you are working on a flat surface it is easy to do.
Apply the tape to the wall, then peel off the wide backing tape. You will be left with the two narrow pieces of tape perfectly spaced apart to define the line.
If you are only trying to create a single straight line to divide the wall from the ceiling, here is a simple method you could try. Draw a chalk line very tightly from one end of the wall to the other. Snap a line, then apply a good quality bleed resistant painters tape along the chalk line. Do not cover the chalk line with the tape, rather apply the tape just above the line. You can then wash the chalk off with a slightly damp cloth before painting.
One thing I read awhile ago and have applied with good results may be applicable here. This answer is more about where you put the line instead of how to get it straight. For getting it straight you may be able to just eyeball it and where you put the line may make it more noticeable. Maybe even use a combination square to make sure the tape is square to other tape that runs perpendicular to it. Looks like an older home and floors and walls my not be square but if you have the paint lines square to themselves I think it will look good.
What I read is you paint the ceiling first, then when you tape the ceiling to paint the walls you leave about a 1/8" gap on the ceiling so that the wall paint goes on the ceiling a bit. All wall/ceiling joints are slightly filleted and nothing is ever perfectly straight. If you try to paint exactly at the line it looks uneven but if you let the paint go up to the ceiling a little bit the edge looks cleaner.
Your walls appear to have a large radius fillet but the principle may help. Here's a quick mockup of the difference. I didn't try to get the lines straight because it's hard to tell what straight would be with the limited view of the photo you posted but to me the right side looks better.
If you don't want to paint the fillet the wall color, another option I think I've seen is to install a small piece of decorative molding on the wall just under the fillet. Use a level to get it straight and you have an easy reference to paint up to. A small bed molding or maybe an egg and dart scribe molding.
The above example uses a 1/2" cove molding to separate the wall and ceiling. Personally I think that looks the best but there's a little more time and money involved. Use a flat ceiling paint to paint the ceiling to reduce the shininess which shows some of the imperfections in the surface that is seen in your photos.
Use a carpenters level and low tack painters tape. Measure your corners from the floor up to the height of your line. Make a few marks in between so that you may use your level to draw a line from corner to corner. Once you've drawn your lines you can start to apply your low tack painters tape following the line by rolling out your tape in one whole strip not peices. cut a small hole at the tip of an inexpensive latex caulk and squeeze a thin bead of caulk along the edge of the tape on the side that will be painted over. Wipe away any excess caulking. You are just trying to create an impenetrable seem of caulk at the tapes edge when it dries. Once you've painted your ceiling or wall you can carefully pull the tape when the paint is dry. make sure you peel the tape back level with itself, pulling back at an angle can cause the paint to peel with the tape as well.
You will thank me later. If the issue is getting a perfect break between colors, use this trick. Paint your base color and let dry. Lay down your tape and paint the same color again. This creates a dam that cannot be penetrated by your top color. Now, paint with your top color and let dry. Remove tape and be amazed at how straight a line it is. No joke, the first time you do it, your jaw will drop.
It will look funny indeed if you put a perfectly straight line on a curve. Simply use a very fine nap roller — say a hot dog roller — and use it on the flat surface to roll out an edge. Never use masking tape for edges! The paint edge should be where the curve stops. Touch it up a very little bit with a brush if it doesn't look right. The less time you spend on it the better it will look. Do what you will but this advice comes from a pro. Good luck!
