Decoupage can be done on many different types of surfaces. I will explain how to prepare each surface, each on a separate post.
If you have any questions, feel free to post them  on the relevant thread.

New Wood must be sanded and finished with a very fine sandpaper before doing your Decoupage. If there are any small dents in the timber it is sometimes possible to lift the grain by placing a small piece of wet sponge over the dent to force the grain to swell. If this does not work the dent will have to be filled.

FILLING DENTS

If the object to going to have an antiqued finish these dents may add character to the piece, but you will have to be the judge as to which dents to fill and which dents to leave visible as every piece is different. Two Part Auto Body filler is what I use to fill dents and gaps. It is durable and very easy to sand back.

If the piece is not going to be painted, and the wood is going to show, then you will need to use a wood putty that matches the colour of your timber to fill the gaps.

The amount of work involved in preparing an old piece depends on its condition. First remove any hardware such as hinges, clasps or hooks but do not fill the holes made by the screws, this way the hardware can be replaced in the exact same position that it came from originally.

If the hardware is old and rusted it can be restored by getting electroplated. Take the screws in as well as they can be plated to match your hardware.

Old paint and varnish must be sanded to a smooth finish. If the paint is pealing it is advisable to remove it completely with paint remover. and then finished as you would new wood.

If the paint is in good condition, wash it with soap and water, rinse and then dry thoroughly.

When the piece is dry, wipe it down with miniral spirits to remove any grease or wax and seal the surface with a water based sealer. . If there are any small dents in the timber it is sometimes possible to lift the grain by placing a small piece of wet sponge over the dent to force the grain to swell. If this does not work the dent will have to be filled.

FILLING DENTS

If the object is going to have an antiqued finish these dents may add character to the piece, but you will have to be the judge as to which dents to fill and which dents to leave visible as every piece is different. Dents and chips can be filled with wood filler. I actually use Two Part Auto Body filler. It is durable and very easy to sand back.

If you are using an old metal piece all the old finish will have to be removed with a good paint and varnish remover. Follow the directions on the can.

If the object is new, and spray painted that does not necessarily mean that it was rust proofed, so the original finish should be removed.

Once the FINISH is removed, apply a good rust removing product which you can get at a hardware store. Follow the directions on he container. Then scrub with fine steel wool #0000 until it is all squeaky clean and all traces of rust are gone.

Now scrub with any good scouring powder, and then dry.
Paint the whole thing with methylated spirits and allow it to evaporate without touching it. This should take a couple of minutes. Remember no touching. If you really must touch it wear gloves, The natural salts on your skin can start rust forming again

Now apply a good metal primer which is available in a variety of colours these days. This can be bought at any hardware store. Apply it in thin coats so remember to pick up some thinner as well. About three thin coats should be enough, Let each coat dry for about a day before applying the next.

When the last coat is dry, sand very lightly to remove any little bumps. Remember the rule, if you can feel it, the eye will see it.

Apply a thin coat of JoSonja's all purpose sealer and let dry.

Apply your background paint colour, thin coats please. I like using JoSonja's background colours which by the way can be mixed to make new colours.

If you need any tips of background colours and effects go to the page on "Background Colours"

Now you are ready to glue on your pictures.

To prepare glazed ceramics or glass surfaces first wash with soap and water and then dry thoroughly.

When dry rub with methylated spirits or denatured alcohol.

Spray on two or three light coats of Matt Acrylic sealer. Let dry thoroughly between each coat.

There are also some new products on the market specially formulated to adhere to glossy surfaces. Check with your local hardware or paint supplier.

Now you can apply you background colour. If you need hints on background finishes go to the "Background Finishes" thread.