Use Protection.

Seriously. Some of the stuff you're going to be working with is NASTY!
Like 'hazardous chemicals' nasty.

Fiberglass resin, for example, is a two-part liquid plastic that consists of a quantity of resin and a smaller quantity of catalyst or hardener.
Adding the hardener to the resin causes the liquid resin to cure to a very hard, yet brittle plastic; the more you deviate from the manufacturer's specification. Using this resin in combination with a weaved-fiberglass cloth yields a reinforced composite material that is strong, lightweight and rigid.

Polyester resin is a particularly dangerous material because of the fumes that it gives off in an unmixed state and during the curing process. Short-term effects of these fumes are dizziness, nausea and skin irritation. Long term effects of overexposure can include nervous system (brain) damage, asphyxiation, and cancer related hazards.

Yeah. The big 'C'.

As for Bondo (which is a brand name, you'll note the same name on the can of resin), it's basically an auto-repair putty, commonly referred to as a 'body filler'. This stuff mixes with with a hardener as well (an organic peroxide), and turns into a putty which then sets and becomes rock-hard.

Then there's the aerosolized glue.
Seriously... what would make it a good idea in ANY universe to inhale that?  o.O

As for precautions and the like... use common sense.
Resin especially has fumes (as stated above)... a mask that covers nose and mouth and filters the air would be a BIG help... as in safety protocols with these products actually require the use of an organic vapour respirator. If and when you start sanding, there will be particulates flying through the air. When you're wearing your safety goggles that cover your eyes completely, watch closely at the particles of the resin... those tiny tiny fibers of fiberglass? You'd be getting those in your eyes... breathing them without a respirator.
It's a bad scene.

Also, this stuff can and will irritate exposed skin... so wear nitrile or latex gloves whenever you handle it... and be sure to wear clothes you EXPECT to wreck... you ARE going to get some of this stuff on your clothes... and they WILL then be wrecked.

If you use anything more powerful than hand-held sandpaper, use tougher gloves and long sleeves... having a power drill tear a slice into your arm or bruise you if it gets out of control.
Yeah... it can happen.
Trust me.

Be cautious. That's all there is to it. The risk to your health for slipping up on your safety precautions or taking shortcuts just isn't worth it.