How the heck do i get sand paper into the inside of this helmet without breaking it. Bondo is as hard as stone lol
Dremel with a stone.
Dremel. You know i have 1 of those somewhere in my mess of a room. Thanks! 
You bondo’d a helmet back together? That sounds really unsafe imho.
Its not for safety its Thor’s helmet lol
Bondo is not the material to use, if you have not guessed that by now. Wood filler or Squadrons White Filler is what you want to fill in uneven surfaces. Bondo is not an adhesive and will crack when flexed. Epoxy is the best adhesive, followed by a thick (not thin or watery) cyanacrolate.
Well whoops. I did join the pieces with 2 part epoxy but i had a few gaps so i was told to use bondo. This helmet isnt meant to bed thou its solid PLA
Actually bondo is a fantastic adhesive in some cases such as pull out strengths of bolts it surpasses all epoxies
I use a corse grit on my Dremel works pretty well
And bondo is very flexible unlike epoxy which is why they use it to repair car bodies
Idk it stuck to PLA pretty well. Just waiting on my cartridge respirator so i can finish sanding this. I applied it without a respirator and i thought for sure i was gonna die lol
Sandblasting may work .
@michael_hughes Bondo does not repair car bodies, it is used to add styling features such as grooves and lines in automotive bodies, panel vans, or smooth out uneven areas or rough patches. Bondo on an ABS or PLA object is too hard and sanding will remove more of the softer plastic than the bondo. Using a dremel tool with a sanding drum will lead to more gouges in the plastic, but if you have to grind it down, that’s the method you would have to use. Using sandpaper and your hand will sand down more unintentional areas than wanted areas. Using the dremel tool you will get kick-off, that’s when the tool grabs onto a peice of bondo and goes skidding across and damages an unwanted area. You can use aluminum tape, or muffler tape to line either side of the bondo to prevent that damage.
@MidnightVisions
Bondo Lightweight Body Filler quickly and easily fills in dents, scrapes, chips, cracks and rust holes. Non-shrinking and permanently durable 2-part filler offers reliable repairs for a wide variety of automotive, home and hobby projects.
First few lines of the product description
@Cristian_Martinez And completely unnecessarily for your project.
Probably lol. Hey this is my first helmet im allowed to make mistakes 
whoah whoah whoah… hang on a minute, this is INSIDE the helmet!
You don’t need to sand it do you cos you’ll be putting in some padding, right? so the padding will sit on and over the seams?
Yeah but i wanna get the rough patches out so they wont poke me through the foam
Ok, so as MidnightVisions says, tape off the plastic around it so when the Dremel skids (and it will skid) you can knock off the peaks, then use a torn off piece of sandpaper to rough down and partially smooth. That should do.
oh, and keep the Dremel set to a low speed, will be easier to control.