Spackle works well in patching holes less than 1/4"
Joint compound works well in larger holes and divots smaller in size than a quarter. There are various grades of joint compound available from Durabond (to cite one manufacturer's trade name for it) 45 to 90 to topping compound. For the most part, the number designations are for how long it takes to dry fully and thus how much working time you have with it. The other types are for how fine a mix of powder you have. Joint compund used for the scratch coat will be a
bit more gritty than topping compound. Joint compount "Lite" for instance is a light weight composition but it needs more reinforcement when used in larger areas.
I recommend using powder joint compound as opposed to ready mixed. This way you can add glue or as much water as you need for your specific project and are ensured of always getting a fresh batch. The ready mix has a limited shelf life and gets especially bad the longer the can is open. If you use it, grab some out of the can and put it into a seperate container or tray. That way, you can close up the main container as soon as possible and any contaminants you have on the wall won't get into your main tub. Once your ready mix gets dry or contaminated, it is about useless to use and won't adhere to the wall well. You can add a
bit of glue to it but that will only help a
bit. It won't spread well once chunky either.
To sand it, you might want to protect what ever is in the area and have someone around with a sparay
bottle to spritz the air with water in preventing the air from filling up with dust that will get everywhere. Also use of a shop vacuum is helpful. Make sure you wear a mask and
safety goggles when sanding it also.
You can help the amount of stick of either spackle or joint compound by adding a
bit of glue to them as said. White glue works in general, so does wood glue for attachment to wood. Sobo or other flexible glues possibly work best in adding strength to your filler because it stays slightly flexable and will to some extent transfer that flexability to the joint compound. Where possible, you might also want to apply some glue directly to the surface you are filling to help it stick better yet. Problem with using glue is that it does not take paint as well as straight spackle or joint compound so you might have to prime your work or determine if the extra strength is necessary as opposed to just joint compound alone. Paint can also be used pre-mixed with the joint compound.
With this method, you can use fiberglass joint tape to reinforce your hole and expand how much area you can
cover.
Epoxy also works well to fill holes. I like using it with fiberglass joint tape or even
friction tape. This will make what you are doing much similar to making a fiberglass joint or coating because it's reinforced. With all these methods, it just depends what it is and how much of it you are filling.
In Bondo there are two types, the normal stuff that is commonly used in scene shops when not using spackle or joint compound for applications that require a
bit of a strong bond or a bond to synthetic materials, and there is the fiberglass reinforced stuff. The fiberglass reinforced Bondo can be used for re-building or building up damaged areas. For instance for some of the hard
flat style walls used for events, the corners were getting damaged if not chewed up and going away. Fiberglass reinforced Bondo can be applied in layers and later sanded to make up a whole new corner on scenery and
props that is just as strong as the original given some support. In this case a 4" chunk was taken out of a corner. We installed some drywall screws into the corner to support and anchor the Bondo, than built up sufficiently enough layers of it that when sanded, and painted, you could not tell there was a broken and missing corner.
You must always use a mask with Bondo or it will make you sick when you apply and sand it.
Some other ther types of materials that work well are water putty and plaster of paris. Both have their advantages depending upon what you are working with.