FDM prints are not fully waterproof because small gaps can exist between layers.
Materials like PETG or ABS improve water resistance compared to PLA.
Post-processing methods like sealing or coating can make FDM parts more waterproof.

Is FDM Waterproof?

FDM parts are not inherently waterproof, but they can be made water-resistant or partially waterproof depending on material and printing settings.

So the correct idea is:

Not fully waterproof by default
Can leak water through microscopic gaps
Can be improved to become water-resistant or near-waterproof

1. Why FDM is NOT naturally waterproof

FDM prints are made by:

depositing layer-by-layer molten plastic
cooling each layer separately

Problem

This creates:

tiny gaps between layers
microscopic pores inside the part

👉 Water can slowly seep through these gaps.

2. Role of Layer Lines

Layer-by-layer structure

Each layer is not perfectly fused
Small voids exist between layers

👉 These act like capillary paths for water leakage

3. Material affects waterproofing

PLA

Not waterproof
Slight porosity
Can absorb moisture over time

ABS

Better water resistance than PLA
Can be smoothed using acetone vapor
👉 Can become almost watertight

PETG (Best for water resistance in FDM)

Naturally water-resistant
Good layer adhesion
Often used for containers

Nylon

Not ideal (absorbs water easily)
Swells when exposed to moisture

TPU

Flexible and water-resistant
Used for seals and gaskets

4. Printing settings affecting waterproofing

1. Higher temperature

Improves layer bonding
Reduces gaps

2. High infill (90–100%)

Reduces internal voids

3. More perimeters (walls)

Thick outer shell prevents leakage

4. Small layer height

Better surface sealing

5. Slower print speed

Improves bonding quality

5. Post-processing methods to make FDM waterproof

1. Epoxy coating

Seals surface completely
Most effective method

2. Acetone smoothing (ABS only)

Melts outer layer slightly
Closes micro-gaps

3. Resin coating

Adds waterproof outer layer

4. Paint or sealant

Spray coatings reduce leakage

5. Heat treatment

Slight melting improves sealing

6. Real-world performance

FDM is suitable for:

Water bottles (temporary use)
Planters
Fluid containers (low pressure)
Drainage models

Not suitable for:

High-pressure pipes
Long-term submerged parts
Industrial fluid systems

7. Comparison with other technologies

Technology	Waterproofing
FDM	Moderate (needs sealing)
SLA	Good but can degrade over time
SLS	Better uniform sealing
Injection molding	Excellent (fully waterproof)

Summary:

👉 FDM is not naturally waterproof due to layer gaps, but:

material choice (PETG best)
high infill
post-processing sealing

👉 can make it effectively waterproof for many applications

Conclusion:

FDM parts are not inherently waterproof because of layer-by-layer deposition, which creates small gaps and porosity. However, using materials like PETG, increasing infill, improving printing parameters, and applying post-processing methods such as epoxy coating can make FDM parts water-resistant or nearly waterproof.