You will have to grind them. An investment for a good home typewriter. use (ie, non-professional) - between BGN 100-150. You sand the paint next to wood, treat it with primer (bezir, which serves as a base. I recommend linen) and varnish on top. If the integrity of the windows is damaged (holes, cracks), it is good to use wood putty. Just make sure its color matches the desired end result. If you are not given money for a grinding machine, you can use sandpaper. You start with the largest (eg 60) and gradually move to smaller ones (60, 80, 100, 120, 150 and 180, 180 is for fine smoothing). Even if there are small scratches or scars, you can use 260-300 sandpaper. The larger the number of sandpaper, the finer the processing. The same goes for sandpaper for a sanding machine. And, don't forget - use materials for external and internal conditions, depending on the joinery. Well this. If you are interested in something else - ask. It's my hobby.
1 LesbiParadise answered
Rent a machine with which you can sand the old layers or you can try by hand as you do not need to remove everything, but enough to catch the new paint. Then take water-based paint (it does not smell at all and is not toxic to children) and rub or best spray, but if the doors are not high quality it is good to change them. ATTENTION - if you spray, be sure to invest in a mask, because the particles are never cleared from the liver. If paint drips somewhere, immediately clean it with a wet cloth, because new water-based paints do not fall easily and do not dissolve with acetone. For example, during communism there were some of the chipboard. My parents recently replaced the old doors and there are some nice and affordable ones, while we restored them because they were high quality, but we hired a carpenter.