In the process of making you will no doubt come across a project that has holes, dents, pin holes, scratches, and so on. These imperfections will need to be filled for painting to give a smooth clean surface. The size of the imperfection and how the final product will be used determines which filler method you should use.
Common Types of Filler
Fills many small areas
Drys ~10 minutes
Fully dry in 2-4 hours
Bondo, made by 3M, is great to fill large dents and holes. 'Bondo' is the brand name of filler putty but is the standard to go to at this point. It is a thick putty that never dries until mixed with a hardender. When fully mixed and left to dry it becomes an easily sandable surface that can be very smooth. Its primary use was auto body work but Bondo has expanded the filler into the 'multi purpose' category with various version of the putty. I've covered 3D printed parts with it and sanded it down for a smooth surface. I've used it to fill deep drill holes on my PVC armor where I have countersunk screws. It's a great multi use filler - but it is NOT flexible.
Bondo comes in a few versions. Autobody, glass, wood filler, high bond, and all-purpose. If you're here to make cosplay props and movie props you will never need to use any of the specialty versions. Regular all-purpose will work. The cream hardener does come with the can, something I was not sure of when I first bought it. However, the cream is white and Bondo is gray so it is very hard to judge how much hardener you have used. This is why I spend the extra money and buy the red cream hardener. It does the same thing as the white, just is easier to see how much has been mixed in as the color changes.
Every time you use Bondo you will need to mix the two parts together. I dump out some putty onto cardboard with some hardener and mix with a paint stick. Bondo starts to dry quickly when mixed properly so it is better to mix in small amounts rather than all the Bondo you will need for a project.
Applying the Bondo is as simple as slapping it on and spreading it around. Sanding Bondo is very easy so you don't have to apply it perfectly smooth before it dries. However, spreading and applying Bondo can be tricky if you don't have the right tools. I used various scrap wood, cardboard, and scrap sintra to apply it at first - then I saw plastic spreaders at the hardware store. These spread Bondo with ease - and dried Bondo can easily be removed from them so they are good for more than one use (just bend them and the Bondo cracks off).
Pour out/scoop out some putty onto a disposable mixing surface. I usually take out a 6 inch wide pile about a inch tall. As you work with Bondo you will learn how much you can personally work with at a time.
Squeeze out a tiny amount of cream hardener onto the putty. You do not need much hardener for a lot of putty. A dime sized drop of hardender will work just fine.
Mix the putty and hardener. The end result will be an evenly colored light pink color putty. Too much hardener will make the putty harden too quickly or make it brittle and soft. Too little hardener will never make the putty dry.
Take your applicator tool (plastic spreader) and spread the mixed putty onto and into your project. The working time is 10 minutes or so depending on how much hardener you used. It is better to have too much putty than too little as you can easily sand and smooth what you have to make it even with the surface.
The mixed Bondo will start to cure in less than 10 minutes. After 30 minutes you can apply more. After one hour, if mixed properly and wasn't applied to thick, you can sand it. I like to wait at least 4 hours to make sure everything is good and solid. Just because the outermost Bondo is dry doesn't necessarily mean the deepest part is fully dry.
When you deem the Bondo hard and solid you can begin sanding. Just about every grit sandpaper will have an affect on Bondo. It can become just as smooth as the surface you are applying it too.
Bondo is ready to paint when it is fully dry. No additional prep is needed. However, as always, primer is recommended.
Spot Putty is also made by 3M and is kind of like premixed Bondo. However, it must be applied in very small amounts on very small holes. 'Spot Putty' is the brand name of glazing putty. It is best used on scratches, pin holes, and minor dings. 3M recommends that you use it on dried body filler (Bondo) but it can be used anywhere. If used correctly it dries very fast and does not need a hardener. Just like Bondo, it is not flexible.
Spot Putty is ready to use out of the tube. It MUST be applied lightly and in small quantities. A dab of Spot Putty will almost never dry. It is much better to put on many light coats than one coat that is potentially too thick. Ideally the spot putty should be noticeably dry minutes after application. Personally I find it easiest to just spread the spot putty with my fingers. It does build up on your fingers though and takes a little work to get off. Alternatively you can use plastic spreaders to apply it.
If applied correctly, Spot Putty is dry enough to sand in 30 minutes. Fully dry not long after that.
Just like Bondo, almost every grit sandpaper will have an affect on Spot Putty. It can become just as smooth as the surface you are applying it too.
Spot Putty is ready to paint when fully dry. No additional prep is needed. However, as always, primer is recommended.
2 in 1 Filler and Sandable Primer Spray by Rust-Oleum is an automotive primer made to fill many imperfections at once. It acts just like regular spray paint but comes out very thick and coarse allowing the paint to fill small gaps.
Use just as you would normal spray paint. See my guide on spray painting. The primer doesn't come out smooth and clean though. This lets the primer fill any gaps and allow for better sanding.
Unlike regular primer and spray paint, this is dry and sandable after 10 minutes in normal conditions.
Very sandable, and even wet sandable. Comes off quickly with low grit.
It's primer, so it is ready for a layer of paint.
Epoxy putty is a two part putty that comes in a tube. When mixed together you get a short amount of working time but when fully cured the end result is as 'tough as steel'. It bonds to just about anything, can be sanded and painted, and is very hard. There are many companies that produce epoxy putties, most notably Loctite. You can find their "Repair Putty" in the adhesives section of your local hardware store. However, it is almost double the price for half as much putty. Epoxy putty is mainly used in plumbing and fixing steel pipes. This is why the good and cheap stuff is in the plumbing section. I found Oatey's 'Fix-it Stick' in the plumbing section of Lowes and never looked back.
Bonus points: This can double as an adhesive, moulding epoxy, and filler.
Put on gloves and open the tube. Cut off as much epoxy putty as you need. Begin to mend and mix the putty together with your hands. The darker middle epoxy and lighter outer epoxy must be evenly mixed to achieve a good hold. When evenly mixed the epoxy will start to get warm. This is when you should apply it. Squish into a hole or gap you have. Remove as much excess as you can as it is less convenient to remove when dry. When done handling the epoxy you can remove your gloves. The epoxy will start to set and harden after 2-5 minutes. After 5 minutes it should not be touched until fully hardened.
I've used this to fill holes in thin plastic. I squish it on the top and bottom and though the hole. When fully dry I sand both sides down and what is left is a piece of epoxy the exact size of the hole. I apply a little glue around the edges to make sure it stays. Looks like a solid piece of plastic when painted.
Starts to harden in 5 minutes.
Handle in an hour.
Fully dry and hard in 15-24 hours.
Due to the epoxy drying much harder than other fillers, it is harder to sand. Sand paper will have no problem sanding it down, but more effort will have to be used than with other filler methods.
Due to its nature it can also be wet sanded, though probably isn't needed.
Epoxy filler is ready to paint when fully dry. No additional prep is needed. However, as always, primer is recommended.