Some Tricks of the Trade

Surface Preparation
Most of the time I paint on wood ,tin or glass surfaces. To have the best look to your finished piece it's best to prep these items before you start to paint your design. For wood I always check for holes, cracks, scratches or gouges. If you find any and usually you do just use a good wood filler. Let dry, once it's dry it's time to sand. Always sand with the grain of the wood.
Caution if your using an electric sander, please use safety equipment.
Wipe with a tack cloth or I'll use a paper towel that I spray with Alcohol. Just do a light wipe to remove the dust from sanding. I like to seal wood with a water based wood sealer. Wood is a porous material so without sealing the wood the grain raises giving you a rough surface. After using a sealer it's time to sand again. I use a very light weigh sand paper or some times the unprinted side of a paper bag will do the trick.
Now I do buy prepped wooden pieces from craft stores you know the nice ones that are wrapped in cellophane, most times I don't have to do anything but unwrap those pieces seal then and start to paint. You pay a little more for them but they are worth it.
To paint a Tin surface again you have to make sure it's nice and clean.
If there are rust spots on a older piece used a metal brush or a light weight sandpaper to remove the rust. Now go wash it with soap and water. Then you can use white vinegar or most times I use alcohol to wipe over the piece to remove any soap residue. Now make sure the piece is good and dry. If this is a new piece of metal I just spray it with alcohol and it wipe dry. I use a water based metal primer, one or two good coats, nice and smooth. Once this is dry your ready to paint.
Now for Glass Prep. Most of the time I buy clear glass items. To use my regular acrylic paints on these pieces I begin with a quick clean again with the alcohol. This will remove any dust or oils from our hands so the frost will stick to the glass evenly. Once it's clean and dry it's time to spray on a glass frost that you can find at most hardware and craft stores. It's important not to get to heavy handed with this spray you'll end up with nasty spots or discolorations that you won't like. Now that the frost is dry you have to spray again with one light coat of matt sealer. If you skip this step you'll be sorry. Been there done that...LOL. Leave to dry for an hour or so and now your ready to paint.
These are just basic steps that will give you a good start to a very nice finished piece.