Sure thing
@norm. Bit of a long read, apologies!
I will start by saying I use the dulux trade water based paints which some people either love or hate. The gloss can be a real pain to use in the warmer weather as it dries very fast. However, so far (11 months on) and my living room has not yellowed at all as far as I can tell. Chose your painting times wisely if using this paint - early mornings at the moment! The colder it is the easier it will be. I have used the dulux oil based many times in my last house but everything went yellow so I was not happy using it again for that to happen again.
Door frames:
It depends on how bad the paint work is already. My house is about 7 years old so luckily there isn't many layers of paint on paint. As such mine wasn't too bad so I gave it all a light sand with 120 grit paper to key it and remove any high spots. I had lots of gaps where the architrave meets the actual door frame so they needed digging out, I carefully use a stanley blade to do this (the idea is to make the gap bigger/neater so the filler can go in to it). You can use a filling knife to drag down the gap, but sometimes it cause more damage to the surrounding paint so not advised.
Once everything is sanded and the gaps are dug out, I fill with a 'decorators caulk'. I leave it for at least 12 hours to dry sometimes a full day depending on if I am going to work the next day (the tube recommends 24 hours for best results). I then undercoat 1 or 2 times depending on if I went down to bare timber in places. My thinking is it needs to be 'perfect' before the gloss goes on. Once undercoat has dried I will gloss it using a 2 inch brush (smaller one to hand for areas closer to carpet or around the sides/top.
With regards to keeping the paint off the carpets, I use masking tape and tuck it in as best I can with a filling knife.
Doors
I love painting doors. I would rather do this than the door frames.
I actually take the door off completely, it makes decorating both the door and frames easier. I remove all the door furniture and put the door on to some heavy duty
sawhorses that I have. My doors have a wood grain effect so to avoid sanding this flat I give the door a VERY light sand with 120 grit paper. Any of the indents where the 6 'panels' are I use scotchbrite to make sure everything is keyed. I then vacuum/wipe with damp cloth and undercoat one side of the door (+tops, edges and bottom) with two coats of undercoat, flip the door over and paint two coats on the other side then gloss that side (+tops, edges and bottom). With my fire doors (very heavy) I will then leave the door for at least 18 hours to dry (it only takes about 2 hours to feel touch dry, but it feels 'sticky' for at least 16-18 hours from my experience. I then flip it over and glossing the final side.
P.S I place some old vinyl coasters under the doors to stop the rough material of the sawhorse from damaging/sticking to the door.
I also clean all the hinges, door catch and catch plate with metal polish as mine all had paint all over them.
Another key point for doing any glossing. Make sure you thoroughly vacuum the areas beforehand. If your painting in the same room you were sanding in, just remember all those fine particles still in the air. Let the dust settle for a few hours first and vacuum the floor.
Products:
Caulk: SX siroflex trade strength caulk acrylic in white (I used it last September and no cracks have appeared so happy with this product so far).
Undercoat:
Dulux trade quick dry
Gloss:
Dulux trade quick dry
Note - B&Q often have buy 2 get 1 free on the paint so look out for that deal if you have a lot to do. Buy more undercoat than gloss if you follow my prep work.
Could you send some pictures of your bad mitres? I would be tempted to fill those gaps with wood glue mixed with fine sawdust and then sand once fully dry! If the gaps are not too big, caulk can work wonders, especially if you use a flat blade filling knife to scrap over. Once they are painted with the rest of the door, you would probably find yourself hard pressed to tell if they are not right.
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Here is my current progress. The hall sides of the door frames have only been undercoated. I will gloss them all together when I have decorated the skirting boards. I've cut the carpet away from the skirting and door frames as it's getting replaced. Awfully dark horrible stuff!