The following tips are quick mind dumps. They are not eco specific and are general and in no particular order.
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When filling walls shine a torch sideways along the wall to show up imperfections.
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When rollering use an extension poll even if you can reach. This will help you cover the surface quicker and more evenly.
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Open up hairline cracks with a knife before filling.
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When filling deep holes in wood, screw screws into the hole to help the filler grip (though if the hole is that big you should probably peace in some wood)
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When painting wood work remove fixtures and fittings. This will help you keep a wet working edge, as you'll be able to work faster.
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As a general rule paint flat faces last. So, if you are painting a door with mouldings, paint the mouldings and edges first. By doing this the most visible part of the surface (the face) doesn't have it's paint finish messed up by the coat to the moulding or edges.
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When painting architraves (door frames) apply the paint a little thinner at the bottom, to to prevent the paint running on to the floor.
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When filling dents on walls which have lining paper on them, apply a spirit based paint to the dent first. This will help prevent the paper lifting when painted or any tears in the paper furring up.
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Prime fillers as you would plaster or wood.
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When painting plaster, thin the first coat of emulsion down by 10 to 20%.
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When applying vinyl silk over a different colour or shade, blot out the old paint film with matt emulsion which is the same colour as the top coat. Vinyl silk has terrible opacity.
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When using step ladders with steps on only one side. Remember this if your standing at the top!
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Wash new rollers out before use to remove loose fluff.
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If you find cutting in difficult, it could be because you are using a cheap brush.
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Apply masking tape to carpets before painting skirting boards.
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When painting architraves remember to dust off the fluff from the top. Even if you don't intent to paint the top lip as the fluff will still get in your brush and paint.
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Fill gaps and joints with Caulk.
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If you get small flies sticking to your paint, allow the paint and flies to dry out, then brush the flies off with a dry bush.
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If you notice runs or curtains on your paint work after it has dried but not hardened, remove with a filling knife (upwards). Then allow to harden, rub down and then repaint the affected area.
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The primer sealers by Zinsser will solve most problem areas, including stains and unsound surfaces.
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Using ladders as opposed to scaffolding will generally not save you money as the work will take longer.
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Make pipes less obvious by painting them in the same colour as the wall or what ever the background is.
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Half a football makes a good mixing bowl for fillers. When the filler is dry you can fold the ball inside out and the filler will fall out.
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cables that run around the top of skirting boards and architraves can really ruin a good paint job particularly when there are many of them, if the cables are loose and if they are covered in old paint. This isn't really a tip but something to be aware of. The best solution is to run them behind the wall. I think you can also get skirting boards with gaps in for routing cables.
I'll add more painting & decorating tips to this page over time

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A good list of Painting &
A good list of Painting & Decorating tips.
Here's one from me...
To make a narrow room look broader, hang striped wallpaper horizontally.
Glasgow Painter & Decorator.