If you wanted more mechanical strength for your 3d prints you are in the right place.

Injecting polyester (fiberglass) resin into the 3d print you will get mechanically stronger parts.
You must use PETG filament plastic because polyester resin chemically bonds to PETG
making parts stronger. PLA is not suitable. PLA filament does not chemically react with polyester.
PLA prints filled with polyester are not stronger.
PETG 3d model filled with polyester can hold 3 times more strength.
Yes, you can use epoxy filling to get even more strength, but polyester is 4 times
cheaper than epoxy. Process is simple. Print your 3d model with PETG filament
and chose 30% gyroid infill. Mix polyester with 1% hardening and inject in your 3d model.
Wait 24 hours and you have super strong 3d print model.

Step 1: Slice 3d Model

In Cura load your 3d model and chose 30% gyroid infill.
Gyroid infill enables polyester even spared throughout whole 3d model.
It will fill every corner of your 3d model. More info: https://en.wikipedia.org/wiki/Gyroid

Step 2: Mix Polyester

Mix polyester with 1% hardener. Use digital scale.
You must be precise about this mix ratio.
Polyester mix heats up as it cures. If you put too much hardener
you risk your 3d model to melt or worse , everything bursts into flame.
Use proper HDPE cup You can use this free tool to calculate how much polyester mix you need. http://3dprinterbuddy.com/ . From final calculation subtract 30 % to get polyester mass in grams.

Polyester mix is viscous enough to be injected with ordinary plastic medical syringe.
Create 2 holes on one end of your model. In one hole inject polyester mix with the
help of medical syringe. When polyester start to come out from the other hole your
3d model is completely filled with polyester mix.

Step 3: Strength Test

Strength test consisted from pulling one side of the 3d model and
measuring max force before it broke down. From the test I have concluded
3d part filled with polyester mix is about 3 times stronger.

It looks like ordinary 3d print with better mechanical properties.

Check out video for detail strength test: