First tip: before painting any trim or door or anything that has been routed with grooves of any kind is to clean up the grooves as much as you can. I usually use a toothbrush and give it a good scrubbing to get out all the bits of debris that have accumulated in there. If you paint the debris in, they will always look a bit rough.
I usually spend a couple of days painting all the trims and doors that I will need for a room that I am working on before even thinking about fitting them. I don't want to have to flit between painting and fitting once I get started.
For any interior woodwork I use a couple of coats of a silk finish emulsion (currently the Valspar sample pots). They are water based so make for easy clean up and will go on easier and smoother than any oil-based paint. I think gloss is too shiny - everything needs scaling down to 1/12th in a dolls house, even the shine.
The wood will need denibbing between coats with the finest abrasive you can get hold of - painters sanding sponges are the best. This is because wood is porous and when you paint or stain it will open up the grain and the surface becomes rough to the touch - these 'nibs' need sanding off - preferably without removing the layer of paint you've just applied.
An absolute must is two coats of paint, sometimes more is needed for a good finish. The deciding factor on trims is not to lose the definition of the grooves in the wood. I actually also 'let down' the paint with a little water (mix well) so it doesn't go on too thickly. You can always add layers of paint if you feel you need them but having to take any off a gloppy finish is beyond a nightmare.
Ideally, before adding trims, your walls should be finished; all ready with their paint or wallpaper. If you are papering the walls, leave a gap above the floor and below the ceiling so that the coving and skirting will have a bit of wooden surface to stick to rather than just being stuck on the wallpaper. It is not so easy to leave a gap for a picture rail or dado rail as they are so thin; generally, I just risk those going on to the paper. You could attach all the trims to the walls first and then paper up to and between them as you would in real life but it is fiddly to do.
It is also probably best to have the floor in place too so that the skirting sits on top of it.
Now, at last, we can fit the trims. Start at the ceiling and and at the back of the room. The first thing to go in is the coving. It is nice to use it for any house because it will hide the, often, ugly join between wall and ceiling. When the coving is in place carry on working your way down the walls with your various trims, preferably without the ceiling light in place.
Measuring at the top of a wall and at the back of a room can be very difficult - a ruler often won't do it and a tape measure wont be as accurate as you'll need for this job. This is how I get round that.....
Take any old scrap of paper - light weight and flexy like magazine paper is good. Place its good straight edge along the join between wall and ceiling and make good sharp creases where the walls meet at each side.
.........sharpen the creases and trim off the ends
Cut the first corner from the piece of trim. The forty-five degree angles will be inward facing. I actually keep the trim in the position it will end up in. In other words, in this photo, we are looking at the face that will stick to the ceiling and I have rough marked how I would want that face to end up. Don't worry about degrees the mitre block will do that for you - just be sure you know which way is what. If you are good at spatial stuff you can be much smarter at working out the easiest way to cut a piece but this works just fine for me.
Take back to the mitre block and cut again. (wrong saw incidentally!!! please ignore that)
Stick the trim in place using wood glue or anything else you favour.
Cut the side pieces. This time you won't need a template. Just cut the angle correctly and put the trim in place and mark where you need to cut off the excess wood at the front of your house - a nice straight cut. Glue in the side pieces. The join will always show however good you are - after all, these are cut edges through a complicated profile made in softish wood and covered in paint.
|before touch up|
Touch up the join with some paint with a fine paintbrush: looks perfect at a distance.
|after touch up|
Here are the three coving pieces in place. The doors need to go in next, followed by the dado and/or picture rails and then the skirting boards.
Oops the floor needs to go in!