Marine Plywood Grades, Sanded Plywood & Specialty Panels

Sanded and specialty plywood is what you buy for boat work, cabinets, shelves, flooring underlayment and painted signs. Getting the panel right gives you the surface the job needs: a smooth appearance face, a floor base that resists dents or an overlay made for graphics. Smoothness matters, but what sits beneath that face matters just as much.

Marine plywood grades: start with AB for boat work

Marine plywood is plywood made for marine applications, including boatbuilding and dock work. The two AB marine plywood choices here are 1/2-inch and 3/4-inch panels, both in 4-by-8-foot sheets.

The letters describe the face and back veneers. A-A puts A-grade veneer on both sides, A-B pairs an A face with a B back, and B-B puts B-grade veneer on both sides. For a project with one main visible face, AB puts the better appearance face where you need it. That is the reason to buy AB rather than choosing by thickness alone.

For boatbuilding marine plywood, separate the grade decision from the thickness decision. The 1/2-inch and 3/4-inch sheets give you two thicknesses in the same AB grade. A 3/4-inch marine plywood panel is thicker; the AB designation still describes its veneers, not an allowable load or support spacing. Keep the thickness matched to the boat’s construction.

For a dock or boathouse project at a Montana lake cabin, think about repeated wetting and drying as well as appearance. Exterior is the bond classification for long-term weather exposure. Marine grade and face grade answer different questions, so choose for the water exposure and the construction together.

AC sanded plywood for cabinets, shelves and project panels

Sanded plywood has its face and back plies sanded smooth during manufacture, with B-grade or better veneer on at least one side. Buy it for work you will see and touch. A cabinet side or shelf deserves an appearance surface from the start.

AC means an A-grade face and C-grade back. BC pairs a B-grade face with a C-grade back. A is the higher appearance grade, which makes AC the first pick for a visible cabinet face. A-grade veneer can still contain repairs; grade is not a promise of an uninterrupted grain pattern.

These AC plywood sizes cover full-sheet work and smaller project panels:

AC sanded plywood thickness	Sheet size	Buying distinction
1/4 inch	4 by 4 feet	A smaller project panel for work that needs a thin sheet
3/8 inch	4 by 8 feet	A full sheet between the 1/4-inch and 1/2-inch choices
1/2 inch	4 by 8 feet	A full sheet for projects calling for 1/2-inch material
5/8 inch	4 by 8 feet	An intermediate thickness between 1/2 inch and 3/4 inch
3/4 inch	4 by 8 feet	The thickest AC choice in this group

For 3/4-inch sanded plywood, start with cabinet and shelving work where your construction calls for that thickness. The 1/2-inch and 3/8-inch sanded plywood choices let you keep thinner components at the thickness the project needs. Shelf length, support and what you put on it still govern the construction; an A face is an appearance grade.

Sanded plywood is good for painting, and you can stain it. Choose the face with the finish in mind. Veneer patches can blend under paint but show a different grain under stain. For stained work, choose a face you want to see before opening the finish can.

You can sand plywood smooth, but use light pressure and ease up at the edges so you preserve the face veneer. Buying a sanded panel puts you closer to the finished surface without making heavy sanding part of the job.

Sanded pine plywood identifies a wood type as well as surface preparation. AC and BC identify veneer grades. Also keep “sanded” and “Sande” separate: sanded describes the surface preparation; Sande names a hardwood face species.

The APA grade distinctions are useful here: touch-sanded means light sanding for uniform thickness, and missed sanding spots are permitted. Buy a sanded appearance panel for visible winter cabinet work in Montana; buy flooring underlayment for the floor beneath it.

Plywood underlayment: buy the construction beneath the smooth face

Plywood underlayment is a special grade installed over a subfloor beneath finish flooring. Its face and inner-ply construction resist dents and punctures from concentrated loads. That is why a smooth sheet of ordinary plywood is not an equivalent flooring purchase. APA C-D Plugged does not provide equivalent face-veneer puncture resistance.

For a remodel over a smooth subfloor, 1/4-inch plywood underlayment is the thin-layer choice. APA allows that thickness in this situation, particularly in remodeling, while its general recommendation is at least 11/32 inch. The 3/8-inch underlayment choice gives you the next thickness above that recommendation. Choose thickness for the existing surface and applied loads, not simply to hide an uneven floor.

The square-edge underlayment choices are 1/4, 3/8, 1/2, 5/8 and 3/4 inch, all in 4-by-8-foot sheets. There is also a 3/4-inch, 4-by-8-foot tongue-and-groove underlayment panel. Keep that underlayment purchase separate from the structural subfloor decision.

For installing APA plywood underlayment, start with a dry, flat subfloor and fix existing squeaks before covering it. Acclimate the panels in the installation rooms for several days, standing on edge and separated. Install with the smooth face up, offset the joints from the subfloor joints, and leave fastener heads flush with or slightly below the surface. Uneven joints and poorly seated fasteners can show through resilient flooring.

Montana’s winter flooring season makes that room acclimation worth planning into the job. Bring the panels into the rooms ahead of the flooring work. For water-damage repairs around Libby following the December 2025 flood, drying the subfloor comes first: a damp base can contribute to nail pops and squeaks beneath the new layer.

Exposure 1 suits construction that will be protected from permanent weather exposure, including delays before enclosure. Exterior suits repeated wetting and drying or long-term weather. During Montana’s fall rush to close in before freeze, that distinction matters. Commonly called CDX, APA Rated Sheathing Exposure 1 is a sheathing choice, not a substitute for underlayment or an Exterior classification.

MDO signboard for painted signs

MDO means medium density overlay, a resin-impregnated fiber surface bonded to plywood under heat and pressure. It is plywood with an overlay, not medium density fiberboard. Buy MDO signboard for the painted-sign job rather than choosing cabinet plywood simply because both surfaces feel smooth.

The two signboard choices are 3/4-inch, 4-by-8-foot panels good on one side or good on both sides. Buy good-one-side signboard for a sign read from one direction. Buy good-two-sides signboard when both faces carry lettering or graphics. The second finished face has a clear purpose on a sign people approach from either direction.

Preserve the overlay during preparation. Careless sanding can roughen or damage the surface you bought for the finish. On an outdoor Montana sign, elevation-related UV and temperature swings also make paint maintenance part of the job; choose the panel for the graphics and plan to maintain the exterior finish.

Tongue-and-groove plywood for the structural subfloor

The 1-1/8-inch, 4-by-8-foot tongue-and-groove plywood panel belongs in the structural floor decision. Buy it for the load-carrying layer beneath the finished room, rather than treating thin underlayment as a replacement for the subfloor.

APA Rated Sturd-I-Floor combines subfloor and underlayment functions beneath carpet and pad. Its tongue-and-groove edges eliminate the need for blocking along the long panel edges. That is a useful construction feature when you are getting a floor down during Montana’s short building season.

Some finishes, including vinyl and ceramic tile, still require additional underlayment over Sturd-I-Floor. Buy the structural panel for the floor structure and the surface layer for the finish flooring. Thickness, support spacing and loads belong in the structural choice; a smooth top face does not settle those questions.

The Pick

Buy AB marine plywood in 1/2-inch or 3/4-inch, 4-by-8-foot sheets for boatbuilding and marine projects, with thickness matched to the construction. Buy AC sanded plywood for visible cabinets, shelves and painted or stained projects; choose the face before staining.

Buy 1/4-inch underlayment for a smooth-subfloor remodel, and choose thicker underlayment for the surface and loads the floor must handle. Buy 3/4-inch MDO signboard good on one side for single-sided signs or good on both sides for two-sided graphics. For the structural floor layer, choose the 1-1/8-inch tongue-and-groove plywood panel as part of the floor’s structural design.

Frequently Asked Questions

What is marine plywood?

Marine plywood is a veneer panel made for marine applications, including boatbuilding and dock work. In an A-B panel, you get an A-grade face and a B-grade back that can have patches or repairs. Put the A face where appearance matters, and allow for more finishing on the B back if both sides will show.

What is plywood underlayment?

Plywood underlayment is a specially constructed layer that goes over your subfloor to prepare it for finish flooring. Its face and inner plies resist dents and punctures from concentrated loads, so a smooth surface alone doesn’t make an ordinary sheet equivalent. For resilient flooring, the underlayment designation matters more than how smooth the panel feels.

How do you install plywood underlayment?

For APA plywood underlayment, acclimate the panels for several days in the installation rooms, standing on edge and separated, then install over a dry subfloor with existing squeaks corrected. Lay the smooth face up, offset the joints from the subfloor joints, and keep fastener heads flush with or slightly below the surface. Construction adhesive between these layers can cause installation or staining problems with some resilient sheet flooring. Joint filler needs to finish curing before sanding and covering, because the flooring slows further curing.

What is sanded plywood?

Sanded plywood has face and back plies sanded smooth during manufacture and B-grade or better veneer on at least one side. The wood veneers run in alternating grain directions through the panel. For a visible surface, check the veneer grade as well as the sanding: touch-sanded panels are lightly sanded for uniform thickness and can still have missed spots.

Can you stain sanded plywood?

Yes, you can stain sanded plywood, but stain can reveal the grain difference between a repair patch and the surrounding veneer. An A-grade face can still contain repairs, so look over the whole face before laying out your cuts. Test your stain on an offcut that includes a patch if you want to see how noticeable that difference will be.

Is sanded plywood good for painting?

Yes, sanded plywood gives you a smooth starting surface for paint. Repairs that stand out under stain can blend into a painted finish, but paint won’t level a rough patch or a raised edge. Deal with those spots before priming, then check the primed surface before applying your finish coats.

Can plywood be sanded smooth?

Yes, you can sand plywood smooth, but the face is a veneer, so heavy sanding can cut through it. Use light pressure and take particular care at edges. If you’re preparing MDO signboard, preserve the overlay: careless sanding can roughen or damage the surface intended for your finish.