A cap nut blocks the thread; an elastomer nut seals against the face. They close different leak paths.

A domed blind end stops anything travelling along the thread and out of the end.

A bonded or captive elastomer compresses against the panel and closes the gap where the bolt enters.

If fluid can reach the thread from the far side, only blocking the thread helps.

If the leak is at the face, only a face seal helps.

Assemblies exposed on both sides often need both, or a cap nut plus thread sealant.

Enough to compress the elastomer visibly and no further - over-tightening destroys the seal with no sign from above.

The seal works by the elastomer flowing into surface irregularities under controlled compression.

Under-tightened, contact pressure is insufficient and the joint leaks while looking assembled.

Over-tightened, the elastomer extrudes from under its retainer and is permanently damaged.

Power tools make over-tightening the usual outcome, so a torque setting is worth using.

Manufacturers publish a torque figure for the seal rather than for the thread, and it is often lower than the thread would take.

Whichever tolerates the fluid, the cleaning regime and the temperature range - they differ enormously.

General-purpose elastomers handle water, weather and mild chemicals.

Fuel, oil and solvent exposure each rule out particular chemistries and require specific ones.

Temperature range is a hard limit at both ends: a seal that is flexible warm may crack when cold.

Ultraviolet exposure degrades some elastomers quickly, which matters on anything outdoors.

Food, potable water and pharmaceutical contact each require certified materials with their own documentation.

Yes, and it is a common and sensible use - it removes a snagging hazard and protects the thread.

A protruding bolt end catches clothing, hands and passing equipment.

Covering it also keeps water and dirt out of the exposed thread, which prevents seizing.

Appearance is a genuine reason too, on visible assemblies and furniture.

The bolt must be the correct length so the nut can tighten without bottoming inside the dome.

A bottomed cap nut clamps nothing while feeling perfectly tight, which is the classic failure with them.

It bottoms inside the dome, so the nut stops turning and clamps nothing.

The dome is closed, so there is a finite depth of usable thread.

A bolt that reaches the end stops the nut before it reaches the joint face.

The resistance feels exactly like a tight joint, which is why it is missed.

Trimming the bolt or adding washers are the usual remedies, with washers preferable since they do not damage the thread.

Two-piece cap nuts and separate push-on caps avoid the problem where bolt length varies.

The nut usually is; the seal usually is not.

An elastomer that has been compressed takes a permanent set and will not seal as well again.

Bonded seals cannot be replaced separately, so the whole nut is renewed.

Captive seals in some designs are replaceable, which makes the nut a longer-term component.

Cap nuts with no elastomer are reusable like any nut, provided the threads are sound.

Where a joint is opened routinely, a design with a replaceable seal is worth the higher initial cost.

Often not - a leak path through the thread or through the joint face needs its own treatment.

The nut seals where it bears; it does nothing about the bolt shank passing through the hole.

Thread sealant, a sealing washer under the head, or a bonded washer on the far side each address a different part of the path.

In pressurised assemblies the joint face itself is normally sealed by a gasket, with the fastener seal as a secondary measure.

Blind holes into a pressurised space need thread sealant regardless of the nut.

Working out where the fluid can actually travel is what decides the combination.