What's inside
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For a painted interior door, poplar is usually the practical choice: it is widely available, machines cleanly and takes paint well after sound preparation. Red alder is also paintable, but its appearance and price make more sense when you want a particular grain or are choosing one species for both painted and stained work. Neither species is automatically ready for a flawless finish. Knots, dents, glue residue and uneven primer can show through paint on either one.
Quick comparison
| Factor | Red alder | Poplar |
|---|---|---|
| Typical appeal | Fine, fairly even grain; often chosen for a warmer natural look | Plain, relatively subtle grain; common for painted millwork |
| Paint performance | Good with proper sanding and primer | Good with proper sanding and primer |
| Common issue | Knots or color variation can complicate a uniform finish | Green, gray or brown streaks may be visible before painting |
| Cost and availability | Can cost more and may be less readily stocked, depending on region | Commonly available in paint-grade stock and door components |
| Best fit | A door where alder is preferred for its wood character or mixed finish plans | A straightforward painted door where price and availability matter |
Which wood should you buy?
For a typical opaque-painted interior door, buy poplar if it is available in the sizes and construction you need at a good price. Its color does not matter much once the door is fully painted, and its grain is generally unobtrusive. It is a sensible option for a paint-grade solid-wood door or components such as stiles and rails.
Choose red alder if the price difference is small, you like working with it, or the door may have a clear-finished side or matching alder trim. Alder is often selected for its relatively consistent grain, but “consistent” does not mean defect-free: inspect pieces for knots, patches and dents. If the door will be painted on every face, paying extra for the wood’s appearance usually buys little.
Compare the finished door, not just the board price. A door’s core and construction affect stability, weight and sound more than the species of its face frame. For a prebuilt door, check the manufacturer’s specification and confirm whether the product is solid wood, engineered construction or a wood-faced composite.
What can go wrong under paint?
Poplar sometimes has greenish or dark streaks. With a fully opaque coating, these are usually cosmetic rather than a reason to reject the door, but stains can bleed through some primers and topcoats. Alder can have knots and color variation too; resinous or discolored spots may telegraph through paint if they are not sealed. On either wood, filled nail holes, glue smears and dents can stand out more than the grain after the finish dries.
Check the door in raking light before finishing. Look across the face from an angle, especially around panel edges, joints and repairs. Glue residue is easy to miss on bare wood and can prevent primer from adhering evenly. Scrape or sand it away rather than trying to cover it with another coat.
Preparation for a smooth painted finish
Start with the door flat and clean. If it is unfinished, sand broad surfaces with 120- or 150-grit paper, then finish with 180 grit. Use a sanding block or a random-orbit sander on flat areas, and sand profiles by hand with a folded sheet or suitable abrasive pad. Avoid rounding sharp edges; paint wears through fastest there. Remove dust with a vacuum and a clean, lint-free cloth.
Fill small dents and nail holes with a paintable wood filler, let it dry as directed, and sand it flush. For deeper defects, use a filler rated for the repair rather than piling on a thin layer that will shrink. Seal knots or suspicious stains with a compatible stain-blocking primer. For a routine paint-grade door, apply a quality wood primer suited to the chosen topcoat; follow the label for sanding and recoat times. A light sand with 220 grit between primer and finish coats helps remove raised fibers and nibs.
Apply two thin finish coats rather than one heavy coat. Heavy paint can pool at panel corners, bridge narrow gaps and leave drips along the lower edges. Keep the door supported and check both sides and all four edges. Follow the paint maker’s instructions for temperature, humidity and cure time; a surface that feels dry may still be soft enough to stick to weatherstripping or a frame.
A stain-blocking wood primer is useful where knots or dark marks might bleed through. For routine sanding, 180-grit sanding sheets are a practical step before primer or between coats, while finer paper is better for light de-nibbing than for leveling major defects.
Glue, joints and construction matter too
If you are making a door, use an adhesive intended for interior woodworking and suitable for the joint and finish schedule. Dry-fit the parts first, and wipe or scrape squeeze-out before it hardens. Dried glue left on the face can create pale, paint-resistant patches. Clamping pressure should close the joint without forcing out so much glue that cleanup becomes difficult.
Solid-wood doors move as humidity changes. Use dry, straight stock and sound joinery, and do not assume that a thicker coat of paint will prevent movement. A poorly built door can develop open joints, cracks or panel problems regardless of whether it is alder or poplar. For many buyers, a well-made, stable door in either species is a better purchase than a cheaper door with loose joints or visible repairs.
A practical buying decision
Buy poplar for a budget-conscious, fully painted interior door when the stock is straight and clean. Buy alder when its availability, grain or use in adjacent woodwork justifies the added cost—not because it is inherently easier to paint. In both cases, inspect the door, remove glue and surface defects, use the right primer, and allow the finish to cure before installing hardware or closing the door against its stops.
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