TL;DR: Swim trunk liners exist for support, modesty, and to keep you from needing underwear in the water. But not all liners are equal. Traditional mesh is cheap and scratchy; compression liners offer support but trap water; brief liners can bunch; and going unlined means wearing something underneath. The sweet spot for most men is a lightweight micro-mesh liner—it drains instantly, dries fast, prevents chafing, and stays out of your way all day.

If you've ever pulled on a brand-new pair of swim trunks and immediately wondered what is this thing inside my shorts, you're not alone. That built-in layer is a liner, and it's one of the most quietly important details in men's swimwear—yet almost nobody talks about what makes one type better than another.

The liner affects how your swim trunks feel when you're dry, how they feel when you're wet, and whether you actually want to keep wearing them three hours after your last swim. Get it wrong and you're dealing with chafing, bunching, a slow-drying swamp, or some uncomfortable combination of all three.

Here's a straightforward breakdown of every liner type you'll encounter, what each one does well, and where each one falls short.

Why Swim Trunks Have Liners in the First Place

Liners serve three basic purposes:

1. Support and containment. They hold everything in place so you don't need to wear underwear underneath.
2. Modesty. Wet fabric clings. A liner adds a second layer so that things stay decent when you step out of the pool.
3. Reduced friction. A good liner sits between your skin and the outer fabric, preventing the kind of direct contact that leads to irritation during a long day.

That last point is the one most guys underestimate. If you've ever experienced swim shorts sticking to your body after a swim, the liner (or lack of a good one) is usually part of the problem.

The Five Liner Types, Compared

1. Traditional Mesh Liner

What it is: The stiff, diamond-patterned netting found in the vast majority of budget and mid-range swim trunks. If you've owned a pair of swim shorts from a big-box retailer, you know this liner.

What it does well:
- Cheap to produce, so it's everywhere
- Provides basic modesty
- Drains water reasonably quickly due to the open weave

Where it falls short:
- Rough against the skin—especially after hours of wear
- The rigid mesh can chafe inner thighs and sensitive areas
- Often feels scratchy and "crunchy" when dry
- Tends to lose shape and pill after repeated washes

Traditional mesh is the liner equivalent of a participation trophy: it technically does the job, but you won't enjoy it. It's the number-one reason guys cut the liner out of their swim trunks entirely—which creates its own problems.

2. Lightweight Micro-Mesh Liner

What it is: A finer, softer evolution of the traditional mesh. Instead of the coarse diamond netting, micro-mesh uses much thinner threads woven more tightly, creating a liner that's smooth to the touch but still highly breathable.

What it does well:
- Soft against the skin, dramatically reducing chafing
- Excellent water drainage—water passes through the fine mesh almost instantly
- Dries very quickly because it holds virtually no water
- Lightweight enough to forget it's there
- Maintains airflow and breathability in hot weather

Where it falls short:
- Less compression and support than a full compression liner (which some guys prefer for active water sports)
- Not as widely available—tends to appear in higher-quality swimwear

Micro-mesh is the sweet spot for most men, especially if you're looking for quick-dry swim trunks designed for all-day wear. It gives you support and modesty without trapping heat or water. Blauw uses a lightweight micro-mesh liner for exactly this reason—it provides chafe-free support while draining so fast that it won't hold you back when you move from the ocean to lunch.

3. Compression Liner

What it is: A fitted, stretchy layer similar to compression shorts or boxer briefs, typically made from a nylon-spandex blend. These are increasingly common in performance-oriented swim trunks and "hybrid" shorts.

What it does well:
- Maximum support and containment—nothing moves
- Excellent for high-intensity water sports (surfing, volleyball, wakeboarding)
- Provides solid modesty coverage
- Reduces inner-thigh friction during movement

Where it falls short:
- Absorbs and retains significantly more water than mesh-based liners
- Slow to dry—the snug, thick fabric acts like a sponge
- Can feel hot and restrictive during long periods out of the water
- Adds bulk under the outer short, especially around the waistband and thighs
- Tends to bunch or ride up over time

Compression liners are popular because they feel secure. But that security comes at a real cost: drying time. If you care about why some trunks dry faster than others, the liner is often the biggest bottleneck—and compression liners are the biggest offenders. A pair of swim trunks with an outer shell that dries in minutes can still leave you damp for an hour or more because the compression liner underneath is holding water against your skin.

4. Brief Liner

What it is: A sewn-in brief, similar to what you'd find inside a traditional men's racing suit or some European-style swim shorts. It's essentially built-in underwear.

What it does well:
- Good support in a minimal design
- Less material than a compression liner, so slightly faster drying
- Works well under very short-inseam trunks where a longer liner would show

Where it falls short:
- Can bunch, shift, or ride up—especially during active swimming
- Doesn't always stay in place when the outer short moves
- Limited coverage can be a modesty concern for some guys
- Quality varies wildly; cheap briefs chafe quickly

Brief liners are fine if they're well-constructed and you don't mind adjusting them occasionally. But they're the most inconsistent liner type in terms of comfort—when they fit well, they're great; when they don't, they're constantly on your mind.

5. Unlined (No Liner)

What it is: Exactly what it sounds like—swim trunks with no built-in liner at all. You'll find this in some European swimwear, board shorts, and certain minimal-design brands.

What it does well:
- Maximum freedom—no second layer to deal with
- Potentially the fastest drying (one less layer to hold water)
- You choose what goes underneath: nothing, underwear, or a separate compression short

Where it falls short:
- No built-in support or modesty layer
- Outer fabric alone may cling or become see-through when wet
- Requires wearing something underneath for most public settings
- Seams and waistband hardware sit directly against skin, which can cause irritation

If you're considering going unlined, our guide on what to wear under swim trunks covers your options in detail—including when it makes sense and what to avoid.

Side-by-Side Comparison

Which Liner Type Is Best for All-Day Wear?

If your swim trunks are only for the pool and you're changing immediately after, almost any liner will do. But if you want swim shorts you can wear all day—morning walk, spontaneous swim, afternoon exploring, dinner—the liner has to perform across all of those situations, not just the swim.

That means it needs to:

• Drain water instantly so the shorts can actually dry fast
• Stay soft against your skin whether you've been walking for two hours or just got out of the ocean
• Not bunch, ride, or shift when you sit, stand, or move
• Breathe in the heat instead of trapping warmth against your body

A lightweight micro-mesh liner checks every one of those boxes. It's why swim trunks built around this liner type—like Blauw's quick-dry swim trunks with their OceanKnit™ fabric and micro-mesh liner—can credibly promise all-day comfort. The outer fabric, made from recycled marine plastic in partnership with the SEAQUAL Initiative, dries in approximately 7 minutes. But that speed would be meaningless if the liner underneath was still soaking wet and chafing. The micro-mesh keeps pace.

The Bottom Line

The liner is one of those details that you never think about until it ruins your day. A bad one chafes, holds water, and makes you want to change the second you leave the beach. A good one disappears entirely—you forget it's there while it quietly does its job.

Traditional mesh was the industry default for decades, not because it was comfortable, but because it was cheap. Compression liners swung to the other extreme—maximum support at the cost of drying speed and breathability. Brief liners are hit-or-miss depending on construction. And going unlined works only if you've planned what goes underneath.

For most guys who want swim trunks they can genuinely live in all summer, a lightweight micro-mesh liner is the clear winner. It's the one design that doesn't force you to choose between comfort, support, and performance.

FAQ

Why do most cheap swim trunks use that scratchy mesh liner?
Traditional mesh netting is inexpensive to manufacture and provides basic water drainage and modesty. It became the industry standard not because it's comfortable, but because it's the cheapest option that technically works. Higher-quality swimwear has moved toward finer micro-mesh for noticeably better comfort.

Can I just cut the liner out of my swim trunks?
You can, but you'll lose the built-in support and modesty layer. You'd then need to wear something underneath—our guide on what to wear under swim trunks covers the best options. A better long-term solution is choosing trunks with a liner that's actually comfortable.

Do compression liners slow down drying time?
Yes, significantly. Compression liners use dense, stretchy fabric that absorbs and retains water much longer than mesh-based liners. Even if the outer shell dries quickly, a compression liner can stay damp for an hour or more.

Is a micro-mesh liner supportive enough for active swimming?
For recreational swimming, pool laps, beach days, and general water activities—absolutely. If you're doing competitive surfing or high-impact water sports where you need maximum lockdown, a compression liner may suit you better. But for the vast majority of guys, micro-mesh provides plenty of support without the drawbacks.

Should I wear underwear under swim trunks with a liner?
Generally, no. The liner is designed to replace underwear. Wearing both adds extra bulk, slows drying dramatically, and can increase chafing. The exception is unlined trunks, which are designed to be paired with whatever layer you prefer.