Prepping to paint

I'm currently working on a scratch built jetpack. Its mostly cardboard and foam board and some PVC on the side for the tanks. What should I use before painting? Is there a base coat of something needed to prevent the cardboard from warping? Theres a bit of bondo on it but not all of it is covered.

Hey there! I made my pack out for sale signs, foam board and PVC tubes also. I layed fiberglass over the sign plastic and foam board to make it sturdy. You can buy the cloth and resin at Home Depot or Lowe's. Its not difficult to put on, just read the instructions. You should do the work outside and buy a good respirator though because the fumes are hazardous... and wear gloves too. I have a thread here for my pack...but it doesn't show the fiberglassing. Hope this helps!

Even scrubbing these shingles can be dangerous as you risk knocking fibers loose. I personally wouldn't be too worried about that though. Wear a light respirator if its a concern to you and make sure to keep it good an wet if you are scrubbing with a brush. Wet fibers don't float in the air.
After that I'd prime with a good 100% acrylic primer and topcoat with a good 100% acrylic paint.